Method for manufacturing semiconductor device

ABSTRACT

To reduce defects in an oxide semiconductor film in a semiconductor device. To improve electrical characteristics of and reliability in the semiconductor device including an oxide semiconductor film. A method for manufacturing a semiconductor device includes the steps of forming a gate electrode and a gate insulating film over a substrate, forming an oxide semiconductor film over the gate insulating film, forming a pair of electrodes over the oxide semiconductor film, forming a first oxide insulating film over the oxide semiconductor film and the pair of electrodes by a plasma CVD method in which a film formation temperature is 280° C. or higher and 400° C. or lower, forming a second oxide insulating film over the first oxide insulating film, and performing heat treatment at a temperature of 150° C. to 400° C. inclusive, preferably 300° C. to 400° C. inclusive, further preferably 320° C. to 370° C. inclusive.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. The present invention relates to a process, a machine,manufacture, or a composition of matter. In particular, the presentinvention relates to, for example, a semiconductor device, a displaydevice, a light-emitting device, a power storage device, a drivingmethod thereof, or a manufacturing method thereof. In particular, thepresent invention relates to a semiconductor device including an oxidesemiconductor, a display device including an oxide semiconductor, or alight-emitting device including an oxide semiconductor, for example. Inparticular, the present invention relates to, for example, asemiconductor device including a transistor and a method formanufacturing the semiconductor device.

2. Description of the Related Art

Transistors used for most flat panel displays typified by a liquidcrystal display device and a light-emitting display device are formedusing silicon semiconductors such as amorphous silicon, single crystalsilicon, and polycrystalline silicon provided over glass substrates.Further, transistors formed using such silicon semiconductors are usedin integrated circuits (ICs) and the like.

In recent years, attention has been drawn to a technique in which,instead of a silicon semiconductor, a metal oxide exhibitingsemiconductor characteristics is used for transistors. Note that in thisspecification, a metal oxide exhibiting semiconductor characteristics isreferred to as an oxide semiconductor.

For example, a technique is disclosed in which a transistor ismanufactured using zinc oxide or an In—Ga—Zn-based oxide as an oxidesemiconductor and the transistor is used as a switching element or thelike of a pixel of a display device (see Patent Documents 1 and 2).

REFERENCES Patent Documents

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

In a transistor including an oxide semiconductor film, a large amount ofoxygen vacancies in the oxide semiconductor film cause poor electricalcharacteristics of the transistor and cause an increase in the amount ofchange in electrical characteristics of the transistor, typically thethreshold voltage due to a change over time or a stress test (e.g., abias-temperature (BT) stress test).

Further, not only vacancies but also impurities, typically impuritiessuch as silicon or carbon which is a constituent element of theinsulating film, cause poor electrical characteristics of the transistorwhen a large amount of the impurities is included in the oxidesemiconductor film.

Thus, an object of one embodiment of the present invention is to reducedefects in an oxide semiconductor film of a semiconductor device or thelike including the oxide semiconductor film. Another object of oneembodiment of the present invention is to reduce the impurityconcentration in an oxide semiconductor film of a semiconductor deviceor the like including the oxide semiconductor film. Another object ofone embodiment of the present invention is to improve electricalcharacteristics of a semiconductor device or the like including an oxidesemiconductor film. Another object of one embodiment of the presentinvention is to improve reliability of a semiconductor device or thelike including an oxide semiconductor film. Another object of oneembodiment of the present invention is to provide a semiconductor deviceor the like with low off-state current. Another object of one embodimentof the present invention is to provide a semiconductor device or thelike with low power consumption. Another object of one embodiment of thepresent invention is to provide a display device or the like capable ofcausing less eyestrain. Another object of one embodiment of the presentinvention is to provide a semiconductor device or the like including atransparent semiconductor film. Another object of one embodiment of thepresent invention is to provide a novel semiconductor device or thelike. Another object of one embodiment of the present invention is toprovide a semiconductor device or the like having excellentcharacteristics. Note that the descriptions of these problems do notdisturb the existence of other problems. Note that in one embodiment ofthe present invention, there is no need to achieve all the objects.Other objects will be apparent from and can be derived from thedescription of the specification, the drawings, the claims, and thelike.

One embodiment of the present invention is a method for manufacturing asemiconductor device which includes the steps of forming a gateelectrode and a gate insulating film over a substrate, forming an oxidesemiconductor film over the gate insulating film, forming a pair ofelectrodes in contact with the oxide semiconductor film withoutperforming heat treatment, forming a first oxide insulating film overthe oxide semiconductor film and the pair of electrodes by a plasma CVDmethod in which a film formation temperature is higher than or equal to280° C. and lower than or equal to 400° C., forming a second oxideinsulating film over the first oxide insulating film, and performingheat treatment at a temperature higher than or equal to 150° C. andlower than or equal to 400° C., preferably higher than or equal to 300°C. and lower than or equal to 400° C., more preferably higher than orequal to 320° C. and lower than or equal to 370° C.

Note that the first oxide insulating film can be formed in such a mannerthat the pressure in a treatment chamber is set to be greater than orequal to 100 Pa and less than or equal to 250 Pa with introduction of asource gas into the treatment chamber and a high-frequency power issupplied to an electrode provided in the treatment chamber.

Further, the second oxide insulating film can be formed in such a mannerthat the substrate placed in the treatment chamber which isvacuum-evacuated is held at a temperature higher than or equal to 180°C. and lower than or equal to 280° C., the pressure in the treatmentchamber is set to be greater than or equal to 100 Pa and less than orequal to 250 Pa with introduction of a source gas into the treatmentchamber, and a high-frequency power greater than or equal to 0.17 W/cm²and less than or equal to 0.5 W/cm² is supplied to the electrodeprovided in the treatment chamber.

Further, a silicon oxide film or a silicon oxynitride film is formed aseach of the first oxide insulating film and the second oxide insulatingfilm with a deposition gas containing silicon and an oxidizing gas as asource gas.

In one embodiment of the present invention, defects in an oxidesemiconductor film of a semiconductor device including the oxidesemiconductor film can be reduced. Further, in one embodiment of thepresent invention, impurities in an oxide semiconductor film of asemiconductor device or the like including the oxide semiconductor filmcan be reduced. Further, in one embodiment of the present invention, theelectrical characteristics of a semiconductor device including an oxidesemiconductor film can be improved. Further, in one embodiment of thepresent invention, reliability of a semiconductor device or the likeincluding an oxide semiconductor film can be improved. Further, in oneembodiment of the present invention, a semiconductor device or the likewith low off-state current can be provided. Further, in one embodimentof the present invention, a semiconductor device or the like with lowpower consumption can be provided. Further, in one embodiment of thepresent invention, a display device or the like capable of causing lesseyestrain can be provided. Further, in one embodiment of the presentinvention, a semiconductor device or the like including a transparentsemiconductor film can be provided. Further, in one embodiment of thepresent invention, a novel semiconductor device or the like can beprovided. Further, in one embodiment of the present invention, asemiconductor device or the like having excellent characteristics can beprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views illustrating oneembodiment of a transistor.

FIGS. 2A to 2D are cross-sectional views illustrating one embodiment ofa method for manufacturing a transistor.

FIGS. 3A to 3C are cross-sectional views each illustrating oneembodiment of a transistor.

FIG. 4 is a cross-sectional view illustrating one embodiment of atransistor.

FIGS. 5A and 5B are cross-sectional views each illustrating oneembodiment of a transistor.

FIGS. 6A and 6B are a top view and cross-sectional views illustratingone embodiment of a transistor, and FIGS. 6C and 6D are cross-sectionalviews illustrating another embodiment of a transistor.

FIGS. 7A and 7B are diagrams each illustrating a band structure of atransistor.

FIG. 8 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIGS. 9A to 9C are a top view and cross-sectional views illustrating oneembodiment of a transistor.

FIGS. 10A to 10C are cross-sectionals views illustrating one embodimentof a method for manufacturing a transistor.

FIG. 11 is a cross-sectional view illustrating one embodiment of atransistor.

FIG. 12 is a cross-sectional view illustrating one embodiment of atransistor.

FIGS. 13A to 13C are a top view and cross-sectional views illustratingone embodiment of a transistor.

FIGS. 14A to 14C are a top view and cross-sectional views illustratingone embodiment of a transistor.

FIGS. 15A to 15C are a block diagram and circuit diagrams illustratingone embodiment of a semiconductor device.

FIG. 16 is a top view illustrating one embodiment of a semiconductordevice.

FIG. 17 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIGS. 18A to 18C are cross-sectionals views illustrating one embodimentof a method for manufacturing a semiconductor device.

FIGS. 19A to 19C are cross-sectionals views illustrating one embodimentof a method for manufacturing a semiconductor device.

FIGS. 20A to 20C are cross-sectionals views illustrating one embodimentof a method for manufacturing a semiconductor device.

FIGS. 21A and 21B are cross-sectionals views illustrating one embodimentof a method for manufacturing a semiconductor device.

FIGS. 22A to 22C are cross-sectionals views illustrating one embodimentof a method for manufacturing a semiconductor device.

FIG. 23 is a top view illustrating one embodiment of a semiconductordevice.

FIG. 24 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIGS. 25A to 25C are cross-sectionals views illustrating one embodimentof a method for manufacturing a semiconductor device.

FIGS. 26A and 26B are cross-sectional views illustrating one embodimentof a method for manufacturing a semiconductor device.

FIG. 27 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIGS. 28A to 28C are cross-sectionals views illustrating one embodimentof a method for manufacturing a semiconductor device.

FIGS. 29A to 29C are cross-sectionals views illustrating one embodimentof a method for manufacturing a semiconductor device.

FIG. 30 shows a nanobeam electron diffraction pattern of an oxidesemiconductor.

FIG. 31 shows a nanobeam electron diffraction pattern of an oxidesemiconductor.

FIGS. 32A to 32C illustrate a touch sensor according to one embodiment.

FIGS. 33A to 33E illustrate structural examples of a touchscreen and anelectronic device according to one embodiment.

FIGS. 34A and 34B are diagrams illustrating a pixel provided with atouch sensor according to one embodiment.

FIGS. 35A to 35C illustrate operations of touch sensors and pixelsaccording to one embodiment.

FIG. 36 is a block diagram showing a structural example of a liquidcrystal display device.

FIG. 37 is a timing chart illustrating one example of a method fordriving a liquid crystal display device.

FIGS. 38A to 38C illustrate electronic devices each including asemiconductor device of one embodiment of the present invention.

FIGS. 39A to 39C illustrate an electronic device including asemiconductor device of one embodiment of the present invention;

FIG. 40 is a diagram showing Vg-Id characteristics of transistors.

FIGS. 41A and 41B are graphs showing the amounts of change in thresholdvoltage and shift value of transistors after BT stress tests and afterBT photostress tests.

FIGS. 42A and 42B illustrate the definition of a threshold voltage and ashift value.

FIGS. 43A and 43B show BT stress test results.

FIG. 44 is a diagram showing TDS measurement results.

FIG. 45 is a diagram showing ESR measurement results.

FIG. 46 is a diagram showing SIMS measurement results.

FIGS. 47A and 47B are model diagrams regarding a process of the releaseof H₂O.

FIG. 48 is a model diagram regarding a process of the release of H₂O.

FIG. 49 shows an energy diagram and a schematic diagram regarding theprocess of the release of H₂O.

FIGS. 50A and 50B show SIMS measurement results.

FIGS. 51A and 51B show SIMS measurement results.

FIGS. 52A and 52B show TDS measurement results.

FIGS. 53A and 53B show Vg-Id characteristics of transistors.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to the accompanying drawings. However, the presentinvention is not limited to the following description and it is easilyunderstood by those skilled in the art that the mode and details can bevariously changed without departing from the scope and spirit of thepresent invention. Therefore, the present invention should not beconstrued as being limited to the description in the followingembodiments and examples. In addition, in the following embodiments andexamples, the same portions or portions having similar functions aredenoted by the same reference numerals or the same hatching patterns indifferent drawings, and description thereof will not be repeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales.

Note that terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

Functions of a “source” and a “drain” are sometimes replaced with eachother when the direction of current flow is changed in circuitoperation, for example. Therefore, the terms “source” and “drain” can beused to denote the drain and the source, respectively, in thisspecification.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, a difference between a potential of onepoint and a reference potential (e.g., a ground potential) is merelycalled a potential or a voltage, and a potential and a voltage are usedas synonymous words in many cases. Thus, in this specification, apotential may be rephrased as a voltage and a voltage may be rephrasedas a potential unless otherwise specified.

In this specification, in the case where an etching step is performedafter a photolithography process, a mask formed in the photolithographyprocess is removed after the etching step.

Embodiment 1

In this embodiment, a semiconductor device which is one embodiment ofthe present invention and a manufacturing method thereof are describedwith reference to drawings.

In a transistor including an oxide semiconductor film, oxygen vacanciesare given as an example of a defect which leads to poor electricalcharacteristics of the transistor. For example, the threshold voltage ofa transistor including an oxide semiconductor film which contains oxygenvacancies in the film easily shifts in the negative direction, and sucha transistor tends to have normally-on characteristics. This is becauseelectric charges are generated owing to oxygen vacancies in the oxidesemiconductor film and the resistance is thus reduced. The transistorhaving normally-on characteristics causes various problems in thatmalfunction is likely to be caused when in operation and that powerconsumption is increased when not in operation. Further, there is aproblem in that the amount of change in electrical characteristics,typically in threshold voltage, of the transistor is increased by changeover time or a stress test.

One factor in generating oxygen vacancies is damage caused in amanufacturing process of a transistor. For example, when an insulatingfilm, a conductive film, or the like is formed over an oxidesemiconductor film by a plasma CVD method or a sputtering method, theoxide semiconductor film might be damaged depending on formationconditions thereof.

Another factor in generating oxygen vacancies is release of oxygen fromthe oxide semiconductor film due to heat treatment. For example, thereis a case where heat treatment is performed to remove impurities such ashydrogen, water, or the like contained in the oxide semiconductor film.However, when the heat treatment is performed with the oxidesemiconductor film exposed, oxygen is released from the oxidesemiconductor film, thereby forming an oxygen vacancy.

Further, not only oxygen vacancies but also impurities such as siliconor carbon which is a constituent element of the insulating film causepoor electrical characteristics of a transistor. Therefore, there is aproblem in that mixing of the impurities into an oxide semiconductorfilm reduces the resistance of the oxide semiconductor film and theamount of change in electrical characteristics, typically in thresholdvoltage, of the transistor is increased by change over time or a stresstest.

Thus, an object of this embodiment is to reduce oxygen vacancies in anoxide semiconductor film having a channel region and the concentrationof impurities in the oxide semiconductor film, in a semiconductor deviceincluding a transistor having the oxide semiconductor film.

Moreover, there is a trend in a commercially available display devicetoward a larger screen, e.g., a 60-inch diagonal screen, and further,the development of a display device is aimed even at a screen size of adiagonal of 120 inches or more. Hence, a glass substrate for a displaydevice has grown in size, e.g., to the 8th generation or more. However,in the case of using a large-sized substrate, because heat treatment isperformed at high temperatures, e.g., at 450° C. or higher, anexpensive, large-sized heating apparatus is needed. Accordingly, themanufacturing cost is increased.

Further, high-temperature heat treatment causes a warp or a shrink ofthe substrate, which leads to a reduction in yield.

Thus, one object of this embodiment is to manufacture a semiconductordevice using heat treatment at a temperature which allows the use of alarge-sized substrate and using a small number of heat treatment steps.

FIGS. 1A to 1C are a top view and cross-sectional views of a transistor50 of a semiconductor device. The transistor 50 shown in FIGS. 1A to 1Cis a channel-etched transistor. FIG. 1A is a top view of the transistor50, FIG. 1B is a cross-sectional view taken along dashed-dotted line A-Bin FIG. 1A, and FIG. 1C is a cross-sectional view taken alongdashed-dotted line C-D in FIG. 1A. Note that in FIG. 1A, a substrate 11,one or more of components of the transistor 50 (e.g., a gate insulatingfilm 17), an oxide insulating film 23, an oxide insulating film 24, anitride insulating film 25, and the like are not illustrated forclarity.

The transistor 50 shown in FIGS. 1B and 1C includes a gate electrode 15provided over the substrate 11. Moreover, the gate insulating film 17over the substrate 11 and the gate electrode 15, an oxide semiconductorfilm 18 overlapping with the gate electrode 15 with the gate insulatingfilm 17 provided therebetween, and a pair of electrodes 21 and 22 beingin contact with the oxide semiconductor film 18 are included.Furthermore, a protective film 26 including the oxide insulating film23, the oxide insulating film 24, and the nitride insulating film 25 isformed over the gate insulating film 17, the oxide semiconductor film18, and the pair of electrodes 21 and 22.

The transistor 50 described in this embodiment includes the oxidesemiconductor film 18. Further, part of the oxide semiconductor film 18serves as a channel region. Furthermore, the oxide insulating film 23 isformed in contact with the oxide semiconductor film 18, and the oxideinsulating film 24 is formed in contact with the oxide insulating film23.

The oxide semiconductor film 18 is typically an In—Ga oxide film, anIn—Zn oxide film, or an In-M-Zn oxide film (M represents Al, Ti, Ga, Y,Zr, La, Ce, Nd, or Hf).

Note that in the case where the oxide semiconductor film 18 is anIn-M-Zn oxide film, the proportions of In and M when summation of In andM is assumed to be 100 atomic % are preferably as follows: the atomicpercentage of In is greater than or equal to 25 atomic % and the atomicpercentage of M is less than 75 atomic %; further preferably, the atomicpercentage of In is greater than or equal to 34 atomic % and the atomicpercentage of M is less than 66 atomic %.

The energy gap of the oxide semiconductor film 18 is 2 eV or more,preferably 2.5 eV or more, more preferably 3 eV or more. With the use ofan oxide semiconductor having such a wide energy gap, the off-statecurrent of the transistor 50 can be reduced.

The thickness of the oxide semiconductor film 18 is greater than orequal to 3 nm and less than or equal to 200 nm, preferably greater thanor equal to 3 nm and less than or equal to 100 nm, more preferablygreater than or equal to 3 nm and less than or equal to 50 nm.

In the case where the oxide semiconductor film 18 is In-M-Zn oxide film(M represents Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf), it is preferablethat the atomic ratio of metal elements of a sputtering target used forforming a film of the In-M-Zn oxide satisfy In≧M and Zn≧M. As the atomicratio of metal elements of such a sputtering target, In:M:Zn=1:1:1 andIn:M:Zn=3:1:2 are preferable. Note that the proportion of the atomicratio of the oxide semiconductor film 18 formed using theabove-described sputtering target varies within a range of ±20% as anerror.

An oxide semiconductor film with low carrier density is used as theoxide semiconductor film 18. For example, an oxide semiconductor filmwhose carrier density is 1×10¹⁷/cm³ or lower, preferably 1×10¹⁵/cm³ orlower, more preferably 1×10¹³/cm³ or lower, much more preferably1×10¹¹/cm³ or lower is used as the oxide semiconductor film 18.

Note that, without limitation to that described above, a material withan appropriate composition may be used depending on requiredsemiconductor characteristics and electrical characteristics (e.g.,field-effect mobility and threshold voltage) of a transistor. Further,in order to obtain required semiconductor characteristics of atransistor, it is preferable that the carrier density, the impurityconcentration, the defect density, the atomic ratio of a metal elementto oxygen, the interatomic distance, the density, and the like of theoxide semiconductor film 18 be set to be appropriate.

Note that it is preferable to use, as the oxide semiconductor film 18,an oxide semiconductor film in which the impurity concentration is lowand density of defect states is low, in which case the transistor canhave more excellent electrical characteristics. Here, the state in whichimpurity concentration is low and density of defect states is low (theamount of oxygen vacancies is small) is referred to as “highly purifiedintrinsic” or “substantially highly purified intrinsic”. A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor has few carrier generation sources, and thus has a lowcarrier density in some cases. Thus, in some cases, a transistorincluding the oxide semiconductor film in which a channel region isformed rarely has a negative threshold voltage (is rarely normally-on).A highly purified intrinsic or substantially highly purified intrinsicoxide semiconductor film has a low density of defect states andaccordingly has few carrier traps in some cases. Further, the highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor film has an extremely low off-state current; even when anelement has a channel width of 1×10⁶ μm and a channel length (L) of 10μm, the off-state current can be less than or equal to the measurementlimit of a semiconductor parameter analyzer, i.e., less than or equal to1×10⁻¹³ A, at a voltage (drain voltage) between a source electrode and adrain electrode of from 1 V to 10 V. Thus, the transistor whose channelregion is formed in the oxide semiconductor film has a small variationin electrical characteristics and high reliability in some cases.Charges trapped by the trap states in the oxide semiconductor film takea long time to be released and may behave like fixed charges. Thus, thetransistor whose channel region is formed in the oxide semiconductorfilm having a high density of trap states has unstable electricalcharacteristics in some cases. Examples of the impurities includehydrogen, nitrogen, alkali metal, and alkaline earth metal.

Hydrogen contained in the oxide semiconductor film reacts with oxygenbonded to a metal atom to be water, and in addition, an oxygen vacancyis formed in a lattice from which oxygen is released (or a portion fromwhich oxygen is released). Due to entry of hydrogen into the oxygenvacancy, an electron serving as a carrier is generated in some cases.Further, in some cases, bonding of part of hydrogen to oxygen bonded toa metal element causes generation of an electron serving as a carrier.Thus, a transistor including an oxide semiconductor which containshydrogen is likely to be normally on.

Accordingly, it is preferable that hydrogen be reduced as much aspossible in the oxide semiconductor film 18. Specifically, the hydrogenconcentration of the oxide semiconductor film 18, which is measured bysecondary ion mass spectrometry (SIMS), is lower than or equal to 5×10¹⁹atoms/cm³, preferably lower than or equal to 1×10¹⁹ atoms/cm³, morepreferably lower than or equal to 5×10¹⁸ atoms/cm³, even more preferablylower than or equal to 1×10¹⁸ atoms/cm³, still more preferably lowerthan or equal to 5×10¹⁷ atoms/cm³, yet still more preferably lower thanor equal to 1×10¹⁶ atoms/cm³.

When silicon or carbon which is one of elements belonging to Group 14 iscontained in the oxide semiconductor film 18, oxygen vacancies areincreased, and the oxide semiconductor film 18 becomes an n-type film.Thus, the concentration of silicon or carbon (the concentration ismeasured by SIMS) of the oxide semiconductor film 18 is lower than orequal to 2×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁷atoms/cm³.

Further, the concentration of alkali metal or alkaline earth metal ofthe oxide semiconductor film 18, which is measured by SIMS, is lowerthan or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generatecarriers when bonded to an oxide semiconductor, in which case theoff-state current of the transistor might be increased. Therefore, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal of the oxide semiconductor film 18.

Further, when containing nitrogen, the oxide semiconductor film 18easily has n-type conductivity by generation of electrons serving ascarriers and an increase of carrier density. Thus, a transistorincluding an oxide semiconductor which contains nitrogen is likely to benormally on. For this reason, nitrogen in the oxide semiconductor filmis preferably reduced as much as possible; the concentration of nitrogenwhich is measured by SIMS is preferably set to, for example, lower thanor equal to 5×10¹⁸ atoms/cm³.

The oxide semiconductor film 18 may have a non-single-crystal structure,for example. The non-single crystal structure includes a c-axis alignedcrystalline oxide semiconductor (CAAC-OS) which is described later, apolycrystalline structure, a microcrystalline structure described later,or an amorphous structure, for example. Among the non-single crystalstructure, the amorphous structure has the highest density of defectlevels, whereas CAAC-OS has the lowest density of defect levels.

The oxide semiconductor film 18 may have an amorphous structure, forexample. An oxide semiconductor film having an amorphous structure has,for example, disordered atomic arrangement and no crystalline component.Alternatively, an oxide film having an amorphous structure has, forexample, an absolutely amorphous structure and has no crystal part.

Note that the oxide semiconductor film 18 may be a mixed film includingtwo or more of the following: a region having an amorphous structure, aregion having a microcrystalline structure, a region having apolycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure. The mixed film includes, for example, two ormore of a region having an amorphous structure, a region having amicrocrystalline structure, a region having a polycrystalline structure,a CAAC-OS region, and a region having a single-crystal structure in somecases. Further, the mixed film has a stacked-layer structure of two ormore of a region having an amorphous structure, a region having amicrocrystalline structure, a region having a polycrystalline structure,a CAAC-OS region, and a region having a single-crystal structure in somecases.

Furthermore, in the transistor 50 described in this embodiment, theoxide insulating film 23 is formed in contact with the oxidesemiconductor film 18, and the oxide insulating film 24 in contact withthe oxide insulating film 23 is formed.

The oxide insulating film 23 is an oxide insulating film through whichoxygen is permeated. Note that the oxide insulating film 23 also servesas a film which relieves damage to the oxide semiconductor film 18 atthe time of forming the oxide insulating film 24 later.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, preferably greater than or equal to 5 nm and less than or equal to50 nm can be used as the oxide insulating film 23. Note that in thisspecification, a “silicon oxynitride film” refers to a film thatcontains oxygen at a higher proportion than nitrogen, and a “siliconnitride oxide film” refers to a film that contains nitrogen at a higherproportion than oxygen.

Further, it is preferable that the amount of defects in the oxideinsulating film 23 be small, typically the spin density of a signalwhich appears at g=2.001 due to a dangling bond of silicon, be lowerthan or equal to 3×10¹⁷ spins/cm³ by ESR measurement. This is because ifthe density of defects in the oxide insulating film 23 is high, oxygenis bonded to the defects and the amount of oxygen that passes throughthe oxide insulating film 23 is decreased.

Further, it is preferable that the amount of defects at the interfacebetween the oxide insulating film 23 and the oxide semiconductor film 18be small, typically the spin density of a signal which appears at g=1.93due to an oxygen vacancy in the oxide semiconductor film 18 be lowerthan or equal to 1×10¹⁷ spins/cm³, more preferably lower than or equalto the lower limit of detection by ESR measurement.

Note that in the oxide insulating film 23, all oxygen entering the oxideinsulating film 23 from the outside does not move to the outside of theoxide insulating film 23 and some oxygen remains in the oxide insulatingfilm 23. Further, movement of oxygen occurs in the oxide insulating film23 in some cases in such a manner that oxygen enters the oxideinsulating film 23 and oxygen contained in the oxide insulating film 23is moved to the outside of the oxide insulating film 23.

When the oxide insulating film through which oxygen is permeated isformed as the oxide insulating film 23, oxygen released from the oxideinsulating film 24 provided over the oxide insulating film 23 can bemoved to the oxide semiconductor film 18 through the oxide insulatingfilm 23.

The oxide insulating film 24 is formed in contact with the oxideinsulating film 23. The oxide insulating film 24 is formed using anoxide insulating film which contains oxygen at a higher proportion thanthe stoichiometric composition. Part of oxygen is released by heatingfrom the oxide insulating film which contains oxygen at a higherproportion than the stoichiometric composition. The oxide insulatingfilm containing oxygen at a higher proportion than the stoichiometriccomposition is an oxide insulating film of which the amount of releasedoxygen converted into oxygen atoms is greater than or equal to 1.0×10¹⁸atoms/cm³, preferably greater than or equal to 3.0×10²⁰ atoms/cm³ in TDSanalysis.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, preferably greater than or equal to 50 nm and less than or equal to400 nm can be used as the oxide insulating film 24.

Further, it is preferable that the amount of defects in the oxideinsulating film 24 be small, typically the spin density of a signalwhich appears at g=2.001 originating from a dangling bond of silicon, belower than 1.5×10¹⁸ spins/cm³, more preferably lower than or equal to1×10¹⁸ spins/cm³ by ESR measurement. Note that the oxide insulating film24 is provided more apart from the oxide semiconductor film 18 than theoxide insulating film 23 is; thus, the oxide insulating film 24 may havehigher defect density than the oxide insulating film 23.

Other details of the transistor 50 are described below.

There is no particular limitation on a material and the like of thesubstrate 11 as long as the material has heat resistance high enough towithstand at least heat treatment performed later. For example, a glasssubstrate, a ceramic substrate, a quartz substrate, or a sapphiresubstrate may be used as the substrate 11. Alternatively, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate made of silicon, silicon carbide, or the like, a compoundsemiconductor substrate made of silicon germanium or the like, an SOIsubstrate, or the like may be used. Still alternatively, any of thesesubstrates provided with a semiconductor element may be used as thesubstrate 11. In the case where a glass substrate is used as thesubstrate 11, a glass substrate having any of the following sizes can beused: the 6th generation (1500 mm×1850 mm), the 7th generation (1870mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation(2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm) Thus, alarge-sized display device can be manufactured.

Alternatively, a flexible substrate may be used as the substrate 11, andthe transistor 50 may be provided directly on the flexible substrate.Alternatively, a separation layer may be provided between the substrate11 and the transistor 50. The separation layer can be used when part orthe whole of a semiconductor device formed over the separation layer iscompleted and separated from the substrate 11 and transferred to anothersubstrate. In such a case, the transistor 50 can be transferred to asubstrate having low heat resistance or a flexible substrate as well.

The gate electrode 15 can be formed using a metal element selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, andtungsten; an alloy containing any of these metal elements as acomponent; an alloy containing any of these metal elements incombination; or the like. Further, one or more metal elements selectedfrom manganese or zirconium may be used. The gate electrode 15 may havea single-layer structure or a stacked structure of two or more layers.For example, a single-layer structure of an aluminum film containingsilicon, a two-layer structure in which a titanium film is stacked overan aluminum film, a two-layer structure in which a titanium film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a titanium nitride film, a two-layerstructure in which a tungsten film is stacked over a tantalum nitridefilm or a tungsten nitride film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, and the like can be given. Alternatively, an alloy film or anitride film which contains aluminum and one or more elements selectedfrom titanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium may be used.

The gate electrode 15 can be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible to have a stacked-layer structure formedusing the above light-transmitting conductive material and the abovemetal element.

Further, an In—Ga—Zn-based oxynitride film, an In—Sn-based oxynitridefilm, an In—Ga-based oxynitride film, an In—Zn-based oxynitride film, aSn-based oxynitride film, an In-based oxynitride film, a film of a metalnitride (such as InN or ZnN), or the like may be provided between thegate electrode 15 and the gate insulating film 17. These films each havea work function higher than or equal to 5 eV, preferably higher than orequal to 5.5 eV, which is higher than the electron affinity of the oxidesemiconductor. Thus, the threshold voltage of the transistor includingan oxide semiconductor can be shifted in the positive direction, andwhat is called a normally-off switching element can be achieved. Forexample, in the case of using an In—Ga—Zn-based oxynitride film, anIn—Ga—Zn-based oxynitride film whose nitrogen concentration is higherthan at least the nitrogen concentration of the oxide semiconductor film18, specifically, an In—Ga—Zn-based oxynitride film whose nitrogenconcentration is higher than or equal to 7 at. % is used.

The gate insulating film 17 can be formed to have a single-layerstructure or a stacked-layer structure using, for example, any ofsilicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, aluminum oxide, hafnium oxide, gallium oxide, and Ga—Zn-basedmetal oxide.

The gate insulating film 17 may be formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced.

The thickness of the gate insulating film 17 is greater than or equal to5 nm and less than or equal to 400 nm, preferably greater than or equalto 10 nm and less than or equal to 300 nm, further preferably greaterthan or equal to 50 nm and less than or equal to 250 nm.

The pair of electrodes 21 and 22 is formed to have a single-layerstructure or a stacked-layer structure including, as a conductivematerial, any of metals such as aluminum, titanium, chromium, nickel,copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungstenor an alloy containing any of these metals as its main component. Forexample, a single-layer structure of an aluminum film containingsilicon, a two-layer structure in which a titanium film is stacked overan aluminum film, a two-layer structure in which a titanium film isstacked over a tungsten film, a two-layer structure in which a copperfilm is formed over a copper-magnesium-aluminum alloy film, athree-layer structure in which a titanium film or a titanium nitridefilm, an aluminum film or a copper film, and a titanium film or atitanium nitride film are stacked in this order, a three-layer structurein which a molybdenum film or a molybdenum nitride film, an aluminumfilm or a copper film, and a molybdenum film or a molybdenum nitridefilm are stacked in this order, and the like can be given. Note that atransparent conductive material containing indium oxide, tin oxide, orzinc oxide may be used.

Further, it is possible to prevent outward diffusion of oxygen from theoxide semiconductor film 18 and entry of hydrogen, water, or the likeinto the oxide semiconductor film 18 from the outside by providing thenitride insulating film 25 having a blocking effect against oxygen,hydrogen, water, alkali metal, alkaline earth metal, and the like overthe oxide insulating film 24. The nitride insulating film is formedusing silicon nitride, silicon nitride oxide, aluminum nitride, aluminumnitride oxide, or the like. Note that instead of the nitride insulatingfilm having a blocking effect against oxygen, hydrogen, water, alkalimetal, alkaline earth metal, and the like, an oxide insulating filmhaving a blocking effect against oxygen, hydrogen, water, and the like,may be provided. As the oxide insulating film having a blocking effectagainst oxygen, hydrogen, water, and the like, aluminum oxide, aluminumoxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttriumoxynitride, hafnium oxide, and hafnium oxynitride can be given.

Next, a method for manufacturing the transistor 50 illustrated in FIGS.1A to 1C is described with reference to FIGS. 2A to 2D.

As illustrated in FIG. 2A, the gate electrode 15 is formed over thesubstrate 11, and the gate insulating film 17 is formed over the gateelectrode 15.

Here, a glass substrate is used as the substrate 11.

A method for forming the gate electrode 15 is described below. First, aconductive film is formed by a sputtering method, a CVD method, anevaporation method, or the like. Then, a mask is formed over theconductive film by a photolithography process. Next, part of theconductive film is etched with the use of the mask to form the gateelectrode 15. After that, the mask is removed.

Note that the gate electrode 15 may be formed by an electrolytic platingmethod, a printing method, an inkjet method, or the like instead of theabove formation method.

Here, a 100-nm-thick tungsten film is formed by a sputtering method.Next, a mask is formed by a photolithography process, and the tungstenfilm is subjected to dry etching with the use of the mask to form thegate electrode 15.

The gate insulating film 17 is formed by a sputtering method, a CVDmethod, an evaporation method, or the like.

In the case where a silicon oxide film, a silicon oxynitride film, or asilicon nitride oxide film is formed as the gate insulating film 17, adeposition gas containing silicon and an oxidizing gas are preferred tobe used as a source gas. Typical examples of the deposition gascontaining silicon include silane, disilane, trisilane, and silanefluoride. As the oxidizing gas, oxygen, ozone, dinitrogen monoxide,nitrogen dioxide, and the like can be given as examples.

Moreover, in the case of forming a gallium oxide film as the gateinsulating film 17, a metal organic chemical vapor deposition (MOCVD)method can be employed.

Next, as illustrated in FIG. 2B, the oxide semiconductor film 18 isformed over the gate insulating film 17.

A formation method of the oxide semiconductor film 18 is describedbelow. An oxide semiconductor film which is to be the oxidesemiconductor film 18 is formed over the gate insulating film 17. Then,after a mask is formed over the oxide semiconductor film by aphotolithography process, the oxide semiconductor film is partly etchedusing the mask. Thus, the oxide semiconductor film 18 subjected toelement isolation as illustrated in FIG. 2B is formed. After that, themask is removed.

The oxide semiconductor film which is to be the oxide semiconductor film18 can be formed by a sputtering method, a coating method, a pulsedlaser deposition method, a laser ablation method, or the like.

In the case where the oxide semiconductor film is formed by a sputteringmethod, a power supply device for generating plasma can be an RF powersupply device, an AC power supply device, a DC power supply device, orthe like as appropriate.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or amixed gas of a rare gas and oxygen is used as appropriate. In the caseof using the mixed gas of a rare gas and oxygen, the proportion ofoxygen is preferably higher than that of a rare gas.

Further, a target may be appropriately selected in accordance with thecomposition of the oxide semiconductor film to be formed.

In order to obtain an intrinsic or substantially intrinsic oxidesemiconductor film, besides the high vacuum evacuation of the chamber, ahighly purification of a sputtering gas is also needed. As an oxygen gasor an argon gas used for a sputtering gas, a gas which is highlypurified to have a dew point of −40° C. or lower, preferably −80° C. orlower, further preferably −100° C. or lower, still further preferably−120° C. or lower is used, whereby entry of moisture or the like intothe oxide semiconductor film can be prevented as much as possible.

Here, a 35-nm-thick In—Ga—Zn oxide film is formed as the oxidesemiconductor film by a sputtering method using an In—Ga—Zn oxide target(In:Ga:Zn=1:1:1). Next, a mask is formed over the oxide semiconductorfilm, and part of the oxide semiconductor film is selectively etched.Thus, the oxide semiconductor film 18 is formed.

Next, as shown in FIG. 2C, the pair of electrodes 21 and 22 is formedwithout heat treatment after the formation of the oxide semiconductorfilm 18.

A method for forming the pair of electrodes 21 and 22 is describedbelow. First, a conductive film is formed by a sputtering method, a CVDmethod, an evaporation method, or the like. Then, a mask is formed overthe conductive film by a photolithography process. Next, the conductivefilm is etched with the use of the mask to form the pair of electrodes21 and 22. After that, the mask is removed.

Here, a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a100-nm-thick titanium film are sequentially stacked by a sputteringmethod. Next, a mask is formed over the titanium film by aphotolithography process and the tungsten film, the aluminum film, andthe titanium film are dry-etched with use of the mask to form the pairof electrodes 21 and 22.

Next, as shown in FIG. 2D, the oxide insulating film 23 is formed overthe oxide semiconductor film 18 and the pair of electrodes 21 and 22.Next, the oxide insulating film 24 is formed over the oxide insulatingfilm 23.

Note that after the oxide insulating film 23 is formed, the oxideinsulating film 24 is preferably formed in succession without exposureto the air. After the oxide insulating film 23 is formed, the oxideinsulating film 24 is formed in succession by adjusting at least one ofthe flow rate of a source gas, pressure, a high-frequency power, and asubstrate temperature without exposure to the air, whereby theconcentration of impurities attributed to the atmospheric component atthe interface between the oxide insulating film 23 and the oxideinsulating film 24 can be reduced and oxygen in the oxide insulatingfilm 24 can be moved to the oxide semiconductor film 18; accordingly,the amount of oxygen vacancies in the oxide semiconductor film 18 can bereduced.

As the oxide insulating film 23, a silicon oxide film or a siliconoxynitride film can be formed under the following conditions: thesubstrate placed in a treatment chamber of a plasma CVD apparatus thatis vacuum-evacuated is held at a temperature higher than or equal to280° C. and lower than or equal to 400° C., the pressure is greater thanor equal to 20 Pa and less than or equal to 250 Pa, preferably greaterthan or equal to 100 Pa and less than or equal to 250 Pa withintroduction of a source gas into the treatment chamber, and ahigh-frequency power is supplied to an electrode provided in thetreatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the oxide insulating film 23. Typical examplesof the deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone,dinitrogen monoxide, nitrogen dioxide, and the like can be given asexamples.

With the use of the above conditions, an oxide insulating film throughwhich oxygen is permeated can be formed as the oxide insulating film 23.Further, by providing the oxide film 19 and the oxide insulating film23, damage to the oxide semiconductor film 18 can be reduced in a stepof forming the oxide insulating film 24 which is formed later.

As for the oxide insulating film 23, a silicon oxide film or a siliconoxynitride film can be formed as the oxide insulating film 23 under thefollowing conditions: the substrate placed in a treatment chamber of aplasma CVD apparatus that is vacuum-evacuated is held at a temperaturehigher than or equal to 280° C. and lower than or equal to 400° C., thepressure is greater than or equal to 100 Pa and less than or equal to250 Pa with introduction of a source gas into the treatment chamber, anda high-frequency power is supplied to an electrode provided in thetreatment chamber.

Under the above film formation conditions, the bonding strength ofsilicon and oxygen becomes strong in the above substrate temperaturerange. Thus, as the oxide insulating film 23, a dense and hard oxideinsulating film through which oxygen is permeated, typically, a siliconoxide film or a silicon oxynitride film of which etching usinghydrofluoric acid of 0.5 wt % at 25° C. is performed at a rate of lowerthan or equal to 10 nm/min, preferably lower than or equal to 8 nm/mincan be formed.

The oxide insulating film 23 is formed while heating is performed; thus,hydrogen, water, or the like contained in the oxide semiconductor film18 can be released in the step. Hydrogen contained in the oxidesemiconductor film 18 is bonded to an oxygen radical formed in plasma toform water. Since the substrate is heated in the step of forming theoxide insulating film 23, water formed by bonding of oxygen and hydrogenis released from the oxide semiconductor film. That is, when the oxideinsulating film 23 is formed by a plasma CVD method, the amount of waterand hydrogen contained in the oxide semiconductor film can be reduced.

Further, time for heating in a state where the oxide semiconductor film18 is exposed can be shortened because heating is performed in a step offorming the oxide insulating film 23. Thus, the amount of oxygenreleased from the oxide semiconductor film by heat treatment can bereduced. That is, the amount of oxygen vacancies in the oxidesemiconductor film can be reduced.

Note that by setting the pressure in the treatment chamber to be greaterthan or equal to 100 Pa and less than or equal to 250 Pa, the amount ofwater contained in the oxide insulating film 23 is reduced; thus,variation in electrical characteristics of the transistor 50 can bereduced and change in threshold voltage can be inhibited.

Further, by setting the pressure in the treatment chamber to be greaterthan or equal to 100 Pa and less than or equal to 250 Pa, damage to theoxide semiconductor film 18 can be reduced when the oxide insulatingfilm 23 is formed, so that the amount of oxygen vacancies contained inthe oxide semiconductor film 18 can be reduced. In particular, when thefilm formation temperature of the oxide insulating film 23 or the oxideinsulating film 24 which is formed later is set to be high, typicallyhigher than 220° C., part of oxygen contained in the oxide semiconductorfilm 18 is released and oxygen vacancies are easily formed. Further,when the film formation conditions for reducing the amount of defects inthe oxide insulating film 24 which is formed later are used to increasereliability of the transistor, the amount of released oxygen is easilyreduced. Thus, it is difficult to reduce oxygen vacancies in the oxidesemiconductor film 18 in some cases. However, by setting the pressure inthe treatment chamber to be greater than or equal to 100 Pa and lessthan or equal to 250 Pa to reduce damage to the oxide semiconductor film18 at the time of forming the oxide insulating film 23, oxygen vacanciesin the oxide semiconductor film 18 can be reduced even when the amountof oxygen released from the oxide insulating film 24 is small.

Note that when the ratio of the amount of the oxidizing gas to theamount of the deposition gas containing silicon is 100 or higher, thehydrogen content in the oxide insulating film 23 can be reduced.Consequently, the amount of hydrogen entering the oxide semiconductorfilm 18 can be reduced; thus, the negative shift in the thresholdvoltage of the transistor can be inhibited.

Here, as the oxide insulating film 23, a 50-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane with a flow rateof 30 sccm and dinitrogen monoxide with a flow rate of 4000 sccm areused as a source gas, the pressure in the treatment chamber is 200 Pa,the substrate temperature is 220° C., and a high-frequency power of 150W is supplied to parallel-plate electrodes with the use of a 27.12 MHzhigh-frequency power source. Under the above conditions, a siliconoxynitride film through which oxygen is permeated can be formed.

As the oxide insulating film 24, a silicon oxide film or a siliconoxynitride film is formed under the following conditions: the substrateplaced in a treatment chamber of the plasma CVD apparatus that isvacuum-evacuated is held at a temperature higher than or equal to 180°C. and lower than or equal to 280° C., preferably higher than or equalto 200° C. and lower than or equal to 240° C., the pressure is greaterthan or equal to 100 Pa and less than or equal to 250 Pa, preferablygreater than or equal to 100 Pa and less than or equal to 200 Pa withintroduction of a source gas into the treatment chamber, and ahigh-frequency power of greater than or equal to 0.17 W/cm² and lessthan or equal to 0.5 W/cm², preferably greater than or equal to 0.25W/cm² and less than or equal to 0.35 W/cm² is supplied to an electrodeprovided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the oxide insulating film 24. Typical examplesof the deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone,dinitrogen monoxide, nitrogen dioxide, and the like can be given asexamples.

As the film formation conditions of the oxide insulating film 24, thehigh-frequency power having the above power density is supplied to thetreatment chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; therefore, theoxygen content in the oxide insulating film 24 becomes higher than thatin the stoichiometric composition. On the other hand, in the film formedat a substrate temperature within the above temperature range, the bondbetween silicon and oxygen is weak, and accordingly, part of oxygen inthe film is released by heat treatment in the later step. Thus, it ispossible to form an oxide insulating film which contains oxygen at ahigher proportion than the stoichiometric composition and from whichpart of oxygen is released by heating. Further, the oxide insulatingfilm 23 is provided over the oxide semiconductor film 18. Accordingly,in the step of forming the oxide insulating film 24, the oxideinsulating film 23 serves as a protective film of the oxidesemiconductor film 18. Consequently, the oxide insulating film 24 can beformed using the high-frequency power having a high power density whiledamage to the oxide semiconductor film 18 is reduced.

Note that in the film formation conditions of the oxide insulating film24, the flow rate of the deposition gas containing silicon relative tothe oxidizing gas can be increased, whereby the amount of defects in theoxide insulating film 24 can be reduced. Typically, it is possible toform an oxide insulating film in which the amount of defects is small,i.e. the spin density of a signal which appears at g=2.001 originatingfrom a dangling bond of silicon is lower than 6×10¹⁷ spins/cm³,preferably lower than or equal to 3×10¹⁷ spins/cm³, more preferablylower than or equal to 1.5×10¹⁷ spins/cm³ by ESR measurement. As aresult, the reliability of the transistor can be improved.

Here, as the oxide insulating film 24, a 400-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane with a flow rateof 200 sccm and dinitrogen monoxide with a flow rate of 4000 sccm areused as the source gas, the pressure in the treatment chamber is 200 Pa,the substrate temperature is 220° C., and the high-frequency power of1500 W is supplied to the parallel-plate electrodes with the use of a27.12 MHz high-frequency power source. Note that a plasma CVD apparatusused here is a parallel-plate plasma CVD apparatus in which theelectrode area is 6000 cm², and the power per unit area (power density)into which the supplied power is converted is 0.25 W/cm².

Next, heat treatment is performed. The heat treatment is performedtypically at a temperature of higher than or equal to 150° C. and lowerthan or equal to 400° C., preferably higher than or equal to 300° C. andlower than or equal to 400° C., more preferably higher than or equal to320° C. and lower than or equal to 370° C.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature of higher than or equal to the strainpoint of the substrate if the heating time is short. Therefore, the heattreatment time can be shortened.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is 20 ppm or less,preferably 1 ppm or less, more preferably 10 ppb or less), or a rare gas(argon, helium, or the like). The atmosphere of nitrogen, oxygen,ultra-dry air, or a rare gas preferably does not contain hydrogen,water, and the like.

By the heat treatment, part of oxygen contained in the oxide insulatingfilm 24 can be moved to the oxide semiconductor film 18, so that theamount of oxygen vacancies contained in the oxide semiconductor film 18can be further reduced.

Further, in the case where water, hydrogen, or the like is contained inthe oxide insulating film 23 and the oxide insulating film 24, when thenitride insulating film 25 having a function of blocking water,hydrogen, and the like is formed later and heat treatment is performed,water, hydrogen, or the like contained in the oxide insulating film 23and the oxide insulating film 24 are moved to the oxide semiconductorfilm 18, so that defects are generated in the oxide semiconductor film18. However, by the heating, water, hydrogen, or the like contained inthe oxide insulating film 23 and the oxide insulating film 24 can bereleased; thus, variation in electrical characteristics of thetransistor 50 can be reduced, and change in threshold voltage can beinhibited.

Note that when the oxide insulating film 24 is formed over the oxideinsulating film 23 while being heated, oxygen can be moved to the oxidesemiconductor film 18 to compensate the oxygen vacancies in the oxidesemiconductor film 18; thus, the heat treatment is not necessarilyperformed.

Here, heat treatment is performed at 350° C. for one hour in anatmosphere of nitrogen and oxygen.

Further, when the pair of electrodes 21 and 22 is formed, the oxidesemiconductor film 18 is damaged by the etching of the conductive film,so that oxygen vacancies are generated on the back channel side (theside of the oxide semiconductor film 18 which is opposite to the sidefacing to the gate electrode 15) of the oxide semiconductor film 18.However, with the use of the oxide insulating film containing oxygen ata higher proportion than the stoichiometric composition as the oxideinsulating film 24, the oxygen vacancies generated on the back channelside can be repaired by heat treatment. By this, defects contained inthe oxide semiconductor film 18 can be reduced, and thus, thereliability of the transistor 50 can be improved.

Next, the nitride insulating film 25 is formed by a sputtering method, aCVD method, or the like.

Note that in the case where the nitride insulating film 25 is formed bya plasma CVD method, the substrate placed in the treatment chamber ofthe plasma CVD apparatus that is vacuum-evacuated is preferably set tobe higher than or equal to 300° C. and lower than or equal to 400° C.,more preferably, higher than or equal to 320° C. and lower than or equalto 370° C., so that a dense nitride insulating film can be formed.

In the case where a silicon nitride film is formed by the plasma CVDmethod as the nitride insulating film 25, a deposition gas containingsilicon, nitrogen, and ammonia are preferably used as a source gas. Asthe source gas, a small amount of ammonia compared to the amount ofnitrogen is used, whereby ammonia is dissociated in the plasma andactivated species are generated. The activated species cleave a bondbetween silicon and hydrogen which are contained in a deposition gascontaining silicon and a triple bond between nitrogen molecules. As aresult, a dense silicon nitride film having few defects, in which a bondbetween silicon and nitrogen is promoted and a bond between silicon andhydrogen is few can be formed. On the other hand, when the amount ofammonia with respect to nitrogen is large in a source gas, cleavage of adeposition gas containing silicon and cleavage of nitrogen are notpromoted, so that a sparse silicon nitride film in which a bond betweensilicon and hydrogen remains and defects are increased is formed.Therefore, in a source gas, a flow ratio of the nitrogen to the ammoniais set to be greater than or equal to 5 and less than or equal to 50,preferably greater than or equal to 10 and less than or equal to 50.

Here, in the treatment chamber of a plasma CVD apparatus, a 50-nm-thicksilicon nitride film is formed by a plasma CVD method in which silanewith a flow rate of 50 sccm, nitrogen with a flow rate of 5000 sccm, andammonia with a flow rate of 100 sccm are used as the source gas, thepressure in the treatment chamber is 100 Pa, the substrate temperatureis 350° C., and high-frequency power of 1000 W is supplied toparallel-plate electrodes with a high-frequency power supply of 27.12MHz. Note that the plasma CVD apparatus is a parallel-plate plasma CVDapparatus in which the electrode area is 6000 cm², and the power perunit area (power density) into which the supplied power is converted is1.7×10⁻¹ W/cm².

By the above-described steps, the protective film 26 including the oxideinsulating film 23, the oxide insulating film 24, and the nitrideinsulating film 25 can be formed.

Next, heat treatment may be performed. The heat treatment is performedtypically at a temperature of higher than or equal to 150° C. and lowerthan or equal to 400° C., preferably higher than or equal to 300° C. andlower than or equal to 400° C., more preferably higher than or equal to320° C. and lower than or equal to 370° C.

Through the above-described process, the transistor 50 can bemanufactured.

In this embodiment, the oxide insulating film is formed by a plasma CVDmethod in which heating is performed at a temperature of higher than orequal to 280° C. and lower than or equal to 400° C. Thus, hydrogen,water, or the like contained in the oxide semiconductor film 18 can bereleased. Further, in the step, the length of heating time in a statewhere the oxide semiconductor film is exposed is short, and even whenthe temperature of the oxide semiconductor film with heat treatment islower than or equal to 400° C., it is possible to manufacture atransistor in which the amount of change in threshold voltage isequivalent to that of a transistor subjected to heat treatment at a hightemperature. Consequently, the manufacturing cost of a semiconductordevice can be reduced.

Further, the oxide insulating film containing oxygen at a higherproportion than the stoichiometric composition is formed to overlap withthe oxide semiconductor film which serves as a channel region, and thus,oxygen in the oxide insulating film can be moved to the oxidesemiconductor film. Consequently, the amount of oxygen vacancies in theoxide semiconductor film can be reduced.

In particular, the oxide insulating film through which oxygen ispermeated is formed between the oxide semiconductor film serving as achannel region and the oxide insulating film which contains oxygen at ahigher proportion than the stoichiometric composition. Thus, damage tothe oxide semiconductor film at the time of forming the oxide insulatingfilm which contains oxygen at a higher proportion than thestoichiometric composition can be suppressed. Consequently, the amountof oxygen vacancies in the oxide semiconductor film can be reduced.

From the above, as for a semiconductor device including an oxidesemiconductor film, a semiconductor device in which the amount ofdefects is reduced can be obtained. Further, as for a semiconductordevice including an oxide semiconductor film, a semiconductor devicewith improved electrical characteristics can be obtained.

<Reaction Between Hydrogen Contained in Oxide Semiconductor Film andExcess Oxygen>

Described below is a reaction between an oxygen radical formed at thetime of forming the oxide insulating film by a plasma CVD method andhydrogen contained in the oxide semiconductor film.

First, a source gas for forming an oxygen radical is described.

A dinitrogen monoxide atmosphere and an oxygen atmosphere are typicalexamples of an atmosphere in which an oxygen radical can be formed.

Reaction enthalpy of a reaction in which an oxygen radical is formed inplasma generated in a dinitrogen monoxide atmosphere is calculated.Gaussian 09 is used for the calculation. As a calculation method, a 2ndorder Moller-Plesset perturbation (MP2) is used. As a basis function,cc-pVDZ for electron correlation is used. The calculation results areshown in Formula 1.

ΔH(N₂O→N₂+O)=E_(tot)(N₂)+E_(tot)(O)−E_(tot)(N₂O)=1.864 eV  [Formula 1]

Reaction enthalpy of a reaction in which an oxygen radical is formed inplasma generated in an oxygen atmosphere is calculated. Gaussian 09 isused for the calculation. As a calculation method, a 2nd orderMoller-Plesset perturbation (MP2) is used. As a basis function, cc-pVDZfor electron correlation is used. The calculation results are shown inFormula 2.

ΔH(O₂→2O)=2E_(tot)(O)−E_(tot)(O₂)=5.032 eV  [Formula 2]

The calculation results shown in Formulae 1 and 2 indicate that anoxygen radical is formed more easily in plasma generated in a dinitrogenmonoxide atmosphere than in plasma generated in an oxygen atmosphere.

Next, a process of releasing H₂O with the use of excess oxygen(hereinafter referred to as exO) bonded to Ga atom or oxygen atomlocated on the surface of the oxide semiconductor film is examined. Asthe oxide semiconductor film, InGaZnO₄ is used.

Here, a calculation regarding a process of the release of H₂O isperformed using a plane model (112 atoms) having a vacuum region in ac-axis direction. The plane model is obtained in the following manner: acrystal structure formed by doubling a primitive cell of an InGaZnO₄crystal in an a-axis direction and a b-axis direction is cut along the(001) plane to have three layers including a (Ga,Zn)O layer on theoutermost surface, an InO₂ layer, and an (Ga,Zn)O layer. FIG. 47Aillustrates a model used for the calculation. In FIG. 47A, excess oxygenbonded to a surface of InGaZnO₄ is denoted by exO. Further, two H atomsare arranged in positions apart from exO. Note that as shown in FIG.47B, exO on the surface of the InGaZnO₄ is stable in terms of energywhen Ga-exO-O is formed. Hence, the structure shown in FIG. 47A is usedas an initial structure of a reaction pathway. The calculationconditions are shown in Table 1.

TABLE 1 Software VASP* Functional PAW Pseudopotential GGA/PBE Cut-offenergy 500 eV K-point 2 × 2 × 1 *Vienna Ab initio Simulation Package

FIG. 48 shows the InGaZnO₄ structure in each step from (0) to (8). Notethat two H atoms are denoted by H1 and H2 in the order in which theycome close to exO.

In steps from (0) to (1), H1 is diffused in the vicinity of exO.

In steps from (1) to (2), H1 is bonded to O (O1) bonded to exO.

In steps from (2) to (3), H1 moves beyond O1.

In steps from (3) to (4), H1 is bonded to exO to form Ga-exO-H1.

In steps from (4) to (5), H2 is bonded to O1.

In steps from (5) to (6), H2 moves beyond O1.

In steps from (6) to (7), H2 is bonded to exO.

In steps from (7) to (8), H₂O formed with H1, exO, and H2 is released.

FIG. 49 shows an energy diagram obtained by calculating energy changefrom the step (1) to the step (8). In the calculation, the structure ofthe step (0) is regarded as a reference of energy (0.00 eV) of thereaction pathway. FIG. 49 also shows a schematic diagram of a reactionamong Ga, O, and H in each step.

It is shown from FIG. 49 that, in the case where exO is bonded to thesurface of InGaZnO₄, energy largely decreases by a reaction that formsH₂O from the exO and H in the InGaZnO₄ and a reaction in which the H₂Ois released. That is, the reactions are exothermic reactions.

Thus, in the case where an oxygen radical included in plasma is bondedto the surface of In—Ga—Zn oxide and exists as exO, an oxygen vacancy isnot formed in the In—Ga—Zn oxide, and H in the In—Ga—Zn oxide reactswith exO, whereby H₂O can be formed. Further, the H₂O can be released.Consequently, the concentration of hydrogen contained in the oxidesemiconductor film is reduced.

Modification Example 1 Regarding Base Insulating Film

In the transistor 50 described in this embodiment, a base insulatingfilm can be provided between the substrate 11 and the gate electrode 15as necessary. As a material of the base insulating film, silicon oxide,silicon oxynitride, silicon nitride, silicon nitride oxide, galliumoxide, hafnium oxide, yttrium oxide, aluminum oxide, aluminumoxynitride, and the like can be given as examples. Note that whensilicon nitride, gallium oxide, hafnium oxide, yttrium oxide, aluminumoxide, or the like is used as a material of the base insulating film, itis possible to suppress diffusion of impurities such as alkali metal,water, and hydrogen into the oxide semiconductor film 18 from thesubstrate 11.

The base insulating film can be formed by a sputtering method, a CVDmethod, or the like.

Modification Example 2 Regarding Gate Insulating Film

In the transistor 50 described in this embodiment, the gate insulatingfilm 17 can have a stacked-layer structure as necessary. Here,structures of the gate insulating film 17 are described with referenceto FIGS. 3A to 3C.

As shown in FIG. 3A, the gate insulating film 17 can have astacked-layer structure in which a nitride insulating film 17 a and anoxide insulating film 17 b are stacked in that order over the gateelectrode 15. When the nitride insulating film 17 a is provided over thegate electrode 15, an impurity, typically hydrogen, nitrogen, alkalimetal, alkaline earth metal, or the like, can be prevented from movingfrom the gate electrode 15 to the oxide semiconductor film 18.

Further, when the oxide insulating film 17 b is provided on the oxidesemiconductor film 18 side, density of defect states at the interfacebetween the gate insulating film 17 and the oxide semiconductor film 18can be reduced. Consequently, a transistor whose electricalcharacteristics are hardly degraded can be obtained. Note that it ispreferable to form, as the oxide insulating film 17 b, an oxideinsulating film containing oxygen at a higher proportion than thestoichiometric composition like the oxide insulating film 24. This isbecause density of defect states at the interface between the gateinsulating film 17 and the oxide semiconductor film 18 can be furtherreduced.

As shown in FIG. 3B, the gate insulating film 17 can have astacked-layer structure in which a nitride insulating film 17 c with fewdefects, a nitride insulating film 17 d with a high blocking propertyagainst hydrogen, and the oxide insulating film 17 b are stacked in thatorder from the gate electrode 15 side. When the nitride insulating film17 c with few defects is provided in the gate insulating film 17, thewithstand voltage of the gate insulating film 17 can be improved.Further, when the nitride insulating film 17 d with a high blockingproperty against hydrogen is provided, hydrogen can be prevented frommoving from the gate electrode 15 and the nitride insulating film 17 cto the oxide semiconductor film 18.

An example of a method for forming the nitride insulating films 17 c and17 d shown in FIG. 3B is described below. First, as the nitrideinsulating film 17 c, a silicon nitride film with few defects is formedby a plasma CVD method in which a mixed gas of silane, nitrogen, andammonia is used as a source gas. Then, as the nitride insulating film 17d, a silicon nitride film in which the hydrogen concentration is low andhydrogen can be blocked is formed by switching the source gas to a mixedgas of silane and nitrogen. By such a formation method, the gateinsulating film 17 having a stacked-layer structure of nitrideinsulating films with few defects and a blocking property againsthydrogen can be formed.

As shown in FIG. 3C, the gate insulating film 17 can have astacked-layer structure in which a nitride insulating film 17 e with ahigh blocking property against an impurity, the nitride insulating film17 c with few defects, the nitride insulating film 17 d with a highblocking property against hydrogen, and the oxide insulating film 17 bare stacked in that order from the gate electrode 15 side. When thenitride insulating film 17 e with a high blocking property against animpurity is provided in the gate insulating film 17, an impurity,typically hydrogen, nitrogen, alkali metal, alkaline earth metal, or thelike, can be prevented from moving from the gate electrode 15 to theoxide semiconductor film 18.

An example of a method for forming the nitride insulating films 17 e, 17c, and 17 d shown in FIG. 3C is described below. First, as the nitrideinsulating film 17 e, a silicon nitride film with a high blockingproperty against an impurity is formed by a plasma CVD method in which amixed gas of silane, nitrogen, and ammonia is used as a source gas.Then, a silicon nitride film with few defects is formed as the nitrideinsulating film 17 c by increasing the flow rate of ammonia. Then, asthe nitride insulating film 17 d, a silicon nitride film in which thehydrogen concentration is low and hydrogen can be blocked is formed byswitching the source gas to a mixed gas of silane and nitrogen. By sucha formation method, the gate insulating film 17 having a stacked-layerstructure of nitride insulating films with few defects and a blockingproperty against an impurity can be formed.

Modification Example 3 Regarding Pair of Electrodes

As for the pair of electrodes 21 and 22 provided in the transistor 50described in this embodiment, it is preferable to use a conductivematerial which is easily bonded to oxygen, such as tungsten, titanium,aluminum, copper, molybdenum, chromium, or tantalum, or an alloythereof. Thus, oxygen contained in the oxide semiconductor film 18 andthe conductive material contained in the pair of electrodes 21 and 22are bonded to each other, so that an oxygen deficient region is formedin the oxide semiconductor film 18. Further, in some cases, part ofconstituent elements of the conductive material that forms the pair ofelectrodes 21 and 22 is mixed into the oxide semiconductor film 18.Consequently, as shown in FIG. 4, low-resistance regions 20 a and 20 bare formed in the vicinity of regions of the oxide semiconductor film 18which are in contact with the pair of electrodes 21 and 22. Thelow-resistance regions 20 a and 20 b are formed between the gateinsulating film 17 and the pair of electrodes 21 and 22 so as to be incontact with the pair of electrodes 21 and 22. Since the low-resistanceregions 20 a and 20 b have high conductivity, contact resistance betweenthe oxide semiconductor film 18 and the pair of electrodes 21 and 22 canbe reduced, and thus, the on-state current of the transistor can beincreased.

Further, the pair of electrodes 21 and 22 may each have a stacked-layerstructure of the conductive material which is easily bonded to oxygenand a conductive material which is not easily bonded to oxygen, such astitanium nitride, tantalum nitride, or ruthenium. With such astacked-layer structure, oxidization of the pair of electrodes 21 and 22can be prevented at the interface between the pair of electrodes 21 and22 and the oxide insulating film 23, so that the increase of theresistance of the pair of electrodes 21 and 22 can be inhibited.

Modification Example 4 Regarding Oxide Semiconductor Film

In the method for manufacturing the transistor 50 in this embodiment, acompound formed by reaction of the oxide semiconductor film 18 can beprovided on the side surface of the oxide semiconductor film 18. Here,description is made with reference to FIGS. 5A and 5B each of which isan enlarged view of the vicinity of the oxide semiconductor film 18 ofthe transistor 50 shown in FIG. 1B.

For example, as shown in FIG. 5A, a compound 18 c formed by reaction ofthe oxide semiconductor film 18 can be provided on the back channel sideof the oxide semiconductor film 18. The compound 18 c can be formed byexposure of the oxide semiconductor film 18 to an alkaline solution suchas tetramethylammonium hydroxide (TMAH) solution or an acidic solutionsuch as phosphoric acid, nitric acid, hydrofluoric acid, hydrochloricacid, sulfuric acid, acetic acid, or oxalic acid after the pair ofelectrodes 21 and 22 is formed.

Note that in the step, part of the oxide semiconductor film 18 is etchedand reacts with the alkaline solution or the acidic solution, and thus,a reactant remains. In the case where the oxide semiconductor film 18 isformed using an In—Ga oxide or an In-M-Zn oxide (M represents Al, Ti,Ga, Y, Zr, La, Ce, Nd, or Hf), In (indium oxide) contained in the oxidesemiconductor film 18 is preferentially removed in the step. Thus, thecompound 18 c is formed in which the proportion of Ga or M to In ishigher than the proportion of Ga or M to In in the oxide semiconductorfilm 18.

The compound 18 c in which the proportion of Ga or M is higher than theproportion of In includes Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf as M andthe proportion of Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf in the atomicratio is higher than the proportion of In in the atomic ratio. Thus, animpurity from the outside can be blocked, and accordingly, the amount ofimpurities which move from the outside to the oxide semiconductor film18 can be reduced. Consequently, a transistor whose threshold voltagehardly fluctuates can be manufactured.

Further, by the above-described treatment, an etching residue betweenthe pair of electrodes 21 and 22 can be removed. Thus, occurrence ofleakage current flowing between the pair of electrodes 21 and 22 can beinhibited.

Further, as shown in FIG. 5B, a compound 18 d can be provided on theside surface of the oxide semiconductor film 18. The compound 18 d canbe formed by performing wet etching treatment using an alkaline solutionsuch as a TMAH solution or an acidic solution such as phosphoric acid,nitric acid, hydrofluoric acid, hydrochloric acid, sulfuric acid, aceticacid, or oxalic acid at the time of forming the oxide semiconductor film18. Alternatively, a compound 18 d can be formed by performing dryetching treatment using a boron trichloride gas and a chlorine gas as anetching gas. Further alternatively, the compound 18 d can be formed byexposure of the oxide semiconductor film 18 to the above-describedsolution after the oxide semiconductor film 18 is formed.

The compound 18 d has a proportion of Ga or M higher than a proportionof In, in a manner similar to that of the compound 18 c. Thus, animpurity from the outside can be blocked by the compound 18 d, andaccordingly, the amount of impurities which move from the outside to theoxide semiconductor film 18 can be reduced. Consequently, a transistorwhose threshold voltage hardly fluctuates can be manufactured.

Modification Example 5 Regarding Oxide Semiconductor Film

In the method for manufacturing the transistor 50 described in thisembodiment, after the pair of electrodes 21 and 22 is formed, the oxidesemiconductor film 18 may be exposed to plasma generated in an oxygenatmosphere, so that oxygen may be supplied to the oxide semiconductorfilm 18. Atmospheres of oxygen, ozone, dinitrogen monoxide, nitrogendioxide, and the like can be given as examples of oxidizing atmospheres.Further, in the plasma treatment, the oxide semiconductor film 18 ispreferably exposed to plasma generated with no bias applied to thesubstrate 11 side. Consequently, the oxide semiconductor film 18 can besupplied with oxygen without being damaged; accordingly, the amount ofoxygen vacancies in the oxide semiconductor film 18 can be reduced.Moreover, impurities, e.g., halogen such as fluorine or chlorineremaining on the surface of the oxide semiconductor film 18 due to theetching treatment can be removed. The plasma treatment is preferablyperformed while heating is performed at a temperature higher than orequal to 300° C. Oxygen in the plasma is bonded to hydrogen contained inthe oxide semiconductor film 18 to form water. Since the substrate isheated, the water is released from the oxide semiconductor film 18.Consequently, the amount of hydrogen and water in the oxidesemiconductor film 18 can be reduced.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

Embodiment 2

In this embodiment, a semiconductor device having a transistor in whichthe amount of defects in an oxide semiconductor film can be furtherreduced as compared to Embodiment 1 is described with reference todrawings. The transistor described in this embodiment is different fromthat in Embodiment 1 in that a multilayer film having an oxidesemiconductor film and oxide in contact with the oxide semiconductorfilm is included.

FIGS. 6A and 6B are a top view and cross-sectional views of a transistor60 included in the semiconductor device. FIG. 6A is a top view of thetransistor 60 and FIG. 6B is a cross-sectional view taken alongdashed-dotted line A-B of FIG. 6A. Note that in FIG. 6A, the substrate11, one or more of components of the transistor 60 (e.g., the gateinsulating film 17), the oxide insulating film 23, the oxide insulatingfilm 24, the nitride insulating film 25, and the like are notillustrated for clarity.

The transistor 60 shown in FIGS. 6A and 6B includes the gate electrode15 provided over the substrate 11. Further, a multilayer film 20overlapping with the gate electrode 15 with the gate insulating film 17provided therebetween, and the pair of electrodes 21 and 22 being incontact with the multilayer film 20 are included. Furthermore, theprotective film 26 including the oxide insulating film 23, the oxideinsulating film 24, and the nitride insulating film 25 is formed overthe gate insulating film 17, the multilayer film 20, and the pair ofelectrodes 21 and 22.

In the transistor 60 described in this embodiment, the multilayer film20 includes the oxide semiconductor film 18 and the oxide film 19. Thatis, the multilayer film 20 has a two-layer structure. Further, part ofthe oxide semiconductor film 18 serves as a channel region. Furthermore,the oxide insulating film 23 is formed in contact with the multilayerfilm 20, and the oxide insulating film 24 is formed in contact with theoxide insulating film 23. That is, the oxide film 19 is provided betweenthe oxide semiconductor film 18 and the oxide insulating film 23.

The oxide film 19 is an oxide film containing one or more elements whichform the oxide semiconductor film 18. Since the oxide film 19 containsone or more elements which form the oxide semiconductor film 18,interface scattering is unlikely to occur at the interface between theoxide semiconductor film 18 and the oxide film 19. Thus, the transistorcan have a high field-effect mobility because the movement of carriersis not hindered at the interface.

The oxide film 19 is typically In—Ga oxide, In—Zn oxide, or In-M-Znoxide (M represents Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf). The energy atthe conduction band bottom of the oxide film 19 is closer to a vacuumlevel than that of the oxide semiconductor film 18 is, and typically,the difference between the energy at the conduction band bottom of theoxide film 19 and the energy at the conduction band bottom of the oxidesemiconductor film 18 is any one of 0.05 eV or more, 0.07 eV or more,0.1 eV or more, and 0.15 eV or more, and any one of 2 eV or less, 1 eVor less, 0.5 eV or less, and 0.4 eV or less. That is, the differencebetween the electron affinity of the oxide film 19 and the electronaffinity of the oxide semiconductor film 18 is any one of 0.05 eV ormore, 0.07 eV or more, 0.1 eV or more, and 0.15 eV or more, and any oneof 2 eV or less, 1 eV or less, 0.5 eV or less, and 0.4 eV or less.

The oxide film 19 preferably contains In because carrier mobility(electron mobility) can be increased.

When the oxide film 19 contains a larger amount of Al, Ti, Ga, Y, Zr,La, Ce, Nd, or Hf in an atomic ratio than the amount of In in an atomicratio, any of the following effects may be obtained: (1) the energy gapof the oxide film 19 is widened; (2) the electron affinity of the oxidefilm 19 decreases; (3) an impurity from the outside is blocked; (4) aninsulating property increases as compared to the oxide semiconductorfilm 18; and (5) oxygen vacancies are less likely to be generated in theoxide film 19 containing a larger amount of Al, Ti, Ga, Y, Zr, La, Ce,Nd, or Hf in an atomic ratio than the amount of In in an atomic ratiobecause Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf is a metal element which isstrongly bonded to oxygen.

In the case where the oxide film 19 is In-M-Zn oxide film, theproportions of In and M when summation of In and M is assumed to be 100atomic % are preferably as follows: the atomic percentage of In is lessthan 50 atomic % and the atomic percentage of M is greater than or equalto 50 atomic %; further preferably, the atomic percentage of In is lessthan 25 atomic % and the atomic percentage of M is greater than or equalto 75 atomic %.

Further, in the case where each of the oxide semiconductor film 18 andthe oxide film 19 is In-M-Zn oxide film (M represents Al, Ti, Ga, Y, Zr,La, Ce, Nd, or Hf), the proportion of M atoms (M represents Al, Ti, Ga,Y, Zr, La, Ce, Nd, or Hf) in the oxide film 19 is higher than that inthe oxide semiconductor film 18. Typically, the proportion of M in eachof the films is 1.5 or more times, preferably twice or more, morepreferably three or more times as high as that in the oxidesemiconductor film 18.

Furthermore, in the case where each of the oxide semiconductor film 18and the oxide film 19 is In-M-Zn-based oxide film (M represents Al, Ti,Ga, Y, Zr, La, Ce, Nd, or Hf), when In:M:Zn=x₁:y₁:z₁ [atomic ratio] issatisfied in the oxide film 19 and In:M:Zn=x₂:y₂:z₂ [atomic ratio] issatisfied in the oxide semiconductor film 18, y₁/x₁ is higher thany₂/x₂. It is preferable that y₁/x₁ be 1.5 or more times as high asy₂/x₂. It is further preferable that y₁/x₁ be twice or more as high asy₂/x₂. It is still further preferable that y₁/x₁ be three or more timesas high as y₂/x₂. In this case, it is preferable that in the oxidesemiconductor film, y₂ be higher than or equal to x₂ because atransistor including the oxide semiconductor film can have stableelectric characteristics. However, when y₂ is larger than or equal tothree or more times x₂, the field-effect mobility of the transistorincluding the oxide semiconductor film is reduced. Accordingly, y₂ ispreferably smaller than three times x₂.

In the case where the oxide semiconductor film 18 is an In-M-Zn oxidefilm (M represents Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf), it ispreferable that the atomic ratio of metal elements of a sputteringtarget used for forming the In-M-Zn oxide film satisfy In≧M and Zn≧M. Asthe atomic ratio of metal elements of such a sputtering target,In:M:Zn=1:1:1 and In:M:Zn=3:1:2 are preferable.

Further, in the case where the oxide film 19 is an In-M-Zn oxide film (Mrepresents Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf), the atomic ratio ofmetal elements of a sputtering target used for forming the In-M-Zn oxidefilm preferably satisfies M>In and Zn>0.5×M, and more preferably, Znalso satisfies Zn>M. As the atomic ratio of metal elements of such asputtering target, In:Ga:Zn=1:3:2, In:Ga:Zn=1:3:4, In:Ga:Zn=1:3:5,In:Ga:Zn=1:3:6, In:Ga:Zn=1:3:7, In:Ga:Zn=1:3:8, In:Ga:Zn=1:3:9,In:Ga:Zn=1:3:10, In:Ga:Zn=1:6:4, In:Ga:Zn=1:6:5, In:Ga:Zn=1:6:6,In:Ga:Zn=1:6:7, In:Ga:Zn=1:6:8, In:Ga:Zn=1:6:9, and In:Ga:Zn=1:6:10 arepreferable.

Note that a proportion of each atom in the atomic ratio of the oxidesemiconductor film 18 and the oxide film 19 varies within a range of±20% as an error.

The oxide film 19 also serves as a film which relieves damage to theoxide semiconductor film 18 at the time of forming the oxide insulatingfilm 24 later.

The thickness of the oxide film 19 is greater than or equal to 3 nm andless than or equal to 100 nm, preferably greater than or equal to 3 nmand less than or equal to 50 nm.

The oxide film 19 may have a non-single-crystal structure, for example,like the oxide semiconductor film 18. The non-single crystal structureincludes a c-axis aligned crystalline oxide semiconductor (CAAC-OS)which is described later, a polycrystalline structure, amicrocrystalline structure described later, or an amorphous structure,for example.

The oxide film 19 may have an amorphous structure, for example. Anamorphous oxide semiconductor film, for example, has disordered atomicarrangement and no crystalline component. Alternatively, an amorphousoxide film is, for example, absolutely amorphous and has no crystalpart.

Note that the oxide semiconductor film 18 and the oxide film 19 may eachbe a mixed film including two or more of the following: a region havingan amorphous structure, a region having a microcrystalline structure, aregion having a polycrystalline structure, a CAAC-OS region, and aregion having a single-crystal structure. The mixed film includes, forexample, two or more of a region having an amorphous structure, a regionhaving a microcrystalline structure, a region having a polycrystallinestructure, a CAAC-OS region, and a region having a single-crystalstructure in some cases. Further, the mixed film has a stacked-layerstructure of two or more of a region having an amorphous structure, aregion having a microcrystalline structure, a region having apolycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure in some cases.

Here, the oxide film 19 is provided between the oxide semiconductor film18 and the oxide insulating film 23. Hence, if trap states are formedbetween the oxide film 19 and the oxide insulating film 23 owing toimpurities and defects, electrons flowing in the oxide semiconductorfilm 18 are less likely to be captured by the trap states because thereis a distance between the trap states and the oxide semiconductor film18. Accordingly, the amount of on-state current of the transistor can beincreased, and the field-effect mobility can be increased. When theelectrons are captured by the trap states, the electrons become negativefixed charges. As a result, a threshold voltage of the transistorvaries. However, by the distance between the oxide semiconductor film 18and the trap states, capture of the electrons by the trap states can bereduced, and accordingly a fluctuation of the threshold voltage can bereduced.

Further, impurities from the outside can be blocked by the oxide film19, and accordingly, the amount of impurities which move from theoutside to the oxide semiconductor film 18 can be reduced. Further, anoxygen vacancy is less likely to be formed in the oxide film 19.Consequently, the impurity concentration and the amount of oxygenvacancies in the oxide semiconductor film 18 can be reduced.

Note that the oxide semiconductor film 18 and the oxide film 19 are notformed by simply stacking each film, but are formed to form a continuousjunction (here, in particular, a structure in which the energy of thebottom of the conduction band is changed continuously between eachfilm). In other words, a stacked-layer structure in which there exist noimpurity which forms a defect level such as a trap center or arecombination center at each interface is provided. If an impurityexists between the oxide semiconductor film 18 and the oxide film 19which are stacked, a continuity of the energy band is damaged, and thecarrier is captured or recombined at the interface and then disappears.

In order to form such a continuous energy band, it is necessary to formfilms continuously without being exposed to air, with use of amulti-chamber deposition apparatus (sputtering apparatus) including aload lock chamber. Each chamber in the sputtering apparatus ispreferably evacuated to be a high vacuum state (to the degree of about5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump suchas a cryopump in order to remove water or the like, which serves as animpurity against the oxide semiconductor film, as much as possible.Alternatively, a turbo molecular pump and a cold trap are preferablycombined so as to prevent a backflow of a gas, especially a gascontaining carbon or hydrogen from an exhaust system to the inside ofthe chamber.

As in a transistor 65 shown in FIG. 6D, a multilayer film 34 overlappingwith the gate electrode 15 with the gate insulating film 17 providedtherebetween, and the pair of electrodes 21 and 22 in contact with themultilayer film 34 may be included.

The multilayer film 34 includes an oxide film 31, the oxidesemiconductor film 18, and the oxide film 19. That is, the multilayerfilm 34 has a three-layer structure. The oxide semiconductor film 18serves as a channel region.

Further, the gate insulating film 17 and the oxide film 31 are incontact with each other. That is, the oxide film 31 is provided betweenthe gate insulating film 17 and the oxide semiconductor film 18.

The multilayer film 34 and the oxide insulating film 23 are in contactwith each other. The oxide insulating film 23 and the oxide insulatingfilm 24 are in contact with each other. That is, the oxide film 19 isprovided between the oxide semiconductor film 18 and the oxideinsulating film 23.

The oxide film 31 can be formed using a material and a formation methodof the oxide film 19 described in Embodiment 1.

It is preferable that the thickness of the oxide film 31 be smaller thanthat of the oxide semiconductor film 18. When the thickness of the oxidefilm 31 is greater than or equal to 1 nm and less than or equal to 5 nm,preferably greater than or equal to 1 nm and less than or equal to 3 nm,the amount of change in threshold voltage of the transistor can bereduced.

In the case where the oxide film 19 is In-M-Zn oxide, the proportions ofIn and M when summation of In and M is assumed to be 100 atomic % arepreferably as follows: the atomic percentage of In is less than 50atomic % and the atomic percentage of M is greater than or equal to 50atomic %; further preferably, the atomic percentage of In is less than25 atomic % and the atomic percentage of M is greater than or equal to75 atomic %.

In the transistor described in this embodiment, the oxide film 19 isprovided between the oxide semiconductor film 18 and the oxideinsulating film 23. Hence, if trap states are formed between the oxidefilm 19 and the oxide insulating film 23 owing to impurities anddefects, electrons flowing in the oxide semiconductor film 18 are lesslikely to be captured by the trap states because there is a distancebetween the trap states and the oxide semiconductor film 18.Accordingly, the amount of on-state current of the transistor can beincreased, and the field-effect mobility can be increased. When theelectrons are captured by the trap states, the electrons become negativefixed charges. As a result, a threshold voltage of the transistorvaries. However, by the distance between the oxide semiconductor film 18and the trap states, capture of the electrons by the trap states can bereduced, and accordingly a fluctuation of the threshold voltage can bereduced.

Further, impurities from the outside can be blocked by the oxide film19, and accordingly, the amount of impurities which move from theoutside to the oxide semiconductor film 18 can be reduced. Further, anoxygen vacancy is less likely to be formed in the oxide film 19.Consequently, the impurity concentration and the amount of oxygenvacancies in the oxide semiconductor film 18 can be reduced.

Further, the oxide film 31 is provided between the gate insulating film17 and the oxide semiconductor film 18, and the oxide film 19 isprovided between the oxide semiconductor film 18 and the oxideinsulating film 23. Thus, it is possible to reduce the concentration ofsilicon or carbon in the vicinity of the interface between the oxidefilm 31 and the oxide semiconductor film 18, the concentration ofsilicon or carbon in the oxide semiconductor film 18, or theconcentration of silicon or carbon in the vicinity of the interfacebetween the oxide film 19 and the oxide semiconductor film 18.Consequently, in the multilayer film 34, the absorption coefficientderived from a constant photocurrent method is lower than 1×10⁻³/cm,preferably lower than 1×10⁻⁴/cm, and thus density of localized levels isextremely low.

Since the transistor 65 having such a structure includes very fewdefects in the multilayer film 34 including the oxide semiconductor film32, the electrical characteristics of the transistor can be improved,and typically, the on-state current can be increased and thefield-effect mobility can be improved. Further, in a BT stress test anda BT photostress test which are examples of a stress test, the amount ofchange in threshold voltage is small, and thus, reliability is high.

<Band Structure of Transistor>

Next, a band structure of the multilayer film 20 provided in thetransistor 60 shown in FIG. 6B is described with reference to FIGS. 7Aand 7B.

Here, for example, In—Ga—Zn oxide having an energy gap of 3.15 eV isused as the oxide semiconductor film 18, and In—Ga—Zn oxide having anenergy gap of 3.5 eV is used as the oxide film 19. The energy gaps canbe measured using a spectroscopic ellipsometer (UT-300 manufactured byHORIBA JOBIN YVON SAS.).

The energy difference between the vacuum level and the top of thevalence band (also called ionization potential) of the oxidesemiconductor film 18 and the energy difference between the vacuum leveland the top of the valence band of the oxide film 19 were 8 eV and 8.2eV, respectively. Note that the energy difference between the vacuumlevel and the valence band top can be measured using an ultravioletphotoelectron spectroscopy (UPS) device (VersaProbe manufactured byULVAC-PHI, Inc.).

Thus, the energy difference between the vacuum level and the bottom ofthe conduction band (also called electron affinity) of the oxidesemiconductor film 18 and the energy gap therebetween of the oxide film19 were 4.85 eV and 4.7 eV, respectively.

FIG. 7A schematically illustrates a part of the band structure of themultilayer film 20. Here, the case where a silicon oxide film isprovided in contact with the multilayer film 20 will be described. InFIG. 7A, EcI1 denotes the energy of the bottom of the conduction band inthe silicon oxide film; EcS1 denotes the energy of the bottom of theconduction band in the oxide semiconductor film 18; EcS2 denotes theenergy of the bottom of the conduction band in the oxide film 19; andEcI2 denotes the energy of the bottom of the conduction band in thesilicon oxide film. Further, EcI1 and EcI2 correspond to the gateinsulating film 17 and the oxide insulating film 23 in FIG. 1B,respectively.

As illustrated in FIG. 7A, there is no energy barrier between the oxidesemiconductor film 18 and the oxide film 19, and the energy level of thebottom of the conduction band gradually changes therebetween. In otherwords, the energy level of the bottom of the conduction band iscontinuously changed. This is because the multilayer film 20 contains anelement contained in the oxide semiconductor film 18 and oxygen istransferred between the oxide semiconductor film 18 and the oxide film19, so that a mixed layer is formed.

As shown in FIG. 7A, the oxide semiconductor film 18 in the multilayerfilm 20 serves as a well and a channel region of the transistorincluding the multilayer film 20 is formed in the oxide semiconductorfilm 18. Note that since the energy of the bottom of the conduction bandof the multilayer film 20 is continuously changed, it can be said thatthe oxide semiconductor film 18 and the oxide film 19 are continuous.

Although trap states due to impurities or defects might be formed in thevicinity of the interface between the oxide film 19 and the oxideinsulating film 23 as shown in FIG. 7A, the oxide semiconductor film 18can be distanced from the trap states owing to existence of the oxidefilm 19. However, when the energy difference between EcS1 and EcS2 issmall, an electron in the oxide semiconductor film 18 might reach thetrap state by passing over the energy difference. By being trapped inthe trap state, a negative fixed charge is generated at the interfacewith the insulating film, whereby the threshold voltage of thetransistor is shifted in the positive direction. Therefore, it ispreferable that the energy difference between EcS1 and EcS2 be 0.1 eV ormore, more preferably 0.15 eV or more, because a change in the thresholdvoltage of the transistor is reduced and stable electricalcharacteristics are obtained.

FIG. 7B schematically illustrates a part of the band structure of themultilayer film 20, which is a variation of the band structure shown inFIG. 7A. Here, a structure where silicon oxide films provided in contactwith the multilayer film 20 is described. In FIG. 7B, EcI1 denotes theenergy of the bottom of the conduction band in the silicon oxide film;EcS1 denotes the energy of the bottom of the conduction band in theoxide semiconductor film 18; and EcI2 denotes the energy of the bottomof the conduction band in the silicon oxide film. Further, EcI1corresponds to the gate insulating film 17 in FIG. 1B, and EcI2corresponds to the oxide insulating film 23 in FIG. 1B.

In the transistor illustrated in FIG. 6B, an upper portion of themultilayer film 20, that is, the oxide film 19 might be etched information of the pair of electrodes 21 and 22. Further, a mixed layer ofthe oxide semiconductor film 18 and the oxide film 19 is likely to beformed on the top surface of the oxide semiconductor film 18 information of the oxide film 19.

For example, when the oxide semiconductor film 18 is an oxidesemiconductor film formed with use of, as a sputtering target, In—Ga—Znoxide whose atomic ratio of In to Ga and Zn is 1:1:1 or In—Ga—Zn oxidewhose atomic ratio of In to Ga and Zn is 3:1:2, and an oxide film 19 isan oxide film formed with use of, as a sputtering target, In—Ga—Zn oxidewhose atomic ratio of In to Ga and Zn is 1:3:2 or In—Ga—Zn oxide whoseatomic ratio of In to Ga and Zn is 1:6:4, the Ga content in the oxidefilm 19 is higher than that in the oxide semiconductor film 18. Thus, aGaOx layer or a mixed layer whose Ga content is higher than that in theoxide semiconductor film 18 can be formed on the top surface of theoxide semiconductor film 18.

For that reason, even in the case where the oxide film 19 is etched, theenergy of the bottom of the conduction band of EcS1 on the EcI2 side isincreased and the band structure shown in FIG. 7B can be obtained insome cases.

As in the band structure shown in FIG. 7B, in observation of a crosssection of a channel region, only the oxide semiconductor film 18 in themultilayer film 20 is apparently observed in some cases. However, amixed layer that contains Ga more than the oxide semiconductor film 18does is formed over the oxide semiconductor film 18 in fact, and thusthe mixed layer can be regarded as a 1.5-th layer. Note that the mixedlayer can be confirmed by analyzing a composition in the upper portionof the oxide semiconductor film 18, when the elements contained in themultilayer film 20 are measured by an EDX analysis, for example. Themixed layer can be confirmed, for example, in such a manner that the Gacontent in the composition in the upper portion of the oxidesemiconductor film 18 is larger than the Ga content in the oxidesemiconductor film 18.

Embodiment 3

In this embodiment, a semiconductor device having a transistor in whichthe amount of defects in an oxide semiconductor film can be furtherreduced and the amount of on-state current of the transistor can beincreased as compared to Embodiments 1 and 2 is described with referenceto drawings. The transistor described in this embodiment is differentfrom that in Embodiment 1 in that an oxide film is provided between theoxide insulating film 23 and the pair of electrodes 21 and 22. Note thatin this embodiment, description is made using Embodiment 1; however,this embodiment can also be applied to Embodiment 2 as appropriate.

FIGS. 9A to 9C are a top view and cross-sectional views of a transistor70 included in the semiconductor device. A top view of the transistor 70is shown in FIG. 9A. A cross-sectional view taken along dashed-dottedline A-B in FIG. 9A is shown in FIG. 9B, and a cross-sectional viewtaken along dashed-dotted line C-D is shown in FIG. 9C. Note that inFIG. 9A, the substrate 11, one or more of components of the transistor70 (e.g., the gate insulating film 17), the oxide insulating film 23,the oxide insulating film 24, the nitride insulating film 25, and thelike are not illustrated for clarity.

The transistor 70 is different from the transistor 50 in that the pairof electrodes 21 and 22 is surrounded by an oxide semiconductor film 18a and an oxide film 19 a. Specifically, the transistor 70 includes theoxide semiconductor film 18 a provided over the gate insulating film 17,the pair of electrodes 21 and 22 provided over the oxide semiconductorfilm 18 a, and the oxide film 19 a provided over the oxide semiconductorfilm 18 a and the pair of electrodes 21 and 22.

The transistor 70 is a transistor in which the contact resistancebetween the oxide semiconductor film 18 a and the pair of electrodes 21and 22 is lower than that of the transistor 60 and the on-state currentis improved as compared to the transistor 60 because the pair ofelectrodes 21 and 22 is in contact with the oxide semiconductor film 18a.

Further, since the pair of electrode 21 and 22 is in contact with theoxide semiconductor film 18 a in the transistor 70, the oxide film 19 acan be thickened without increase of the contact resistance between theoxide semiconductor film 18 a and the pair of electrodes 21 and 22.Thus, it is possible to inhibit formation of a trap state, which occursdue to plasma damage at the time of forming the protective film 26,mixing of a constituent element of the protective film 26, or the like,in the vicinity of the interface between the oxide semiconductor film 18a and the oxide film 19 a. That is, the transistor 70 can achieve bothimprovement of on-state current and reduction of change in thresholdvoltage.

A method for manufacturing the transistor 70 is described with referenceto FIGS. 10A to 10C. First, in a manner similar to that of FIG. 2A, thegate electrode and the gate insulating film 17 are formed over thesubstrate 11.

Next, an oxide semiconductor film 28 which is to be the oxidesemiconductor film 18 a is formed, and then, the pair of electrodes 21and 22 is formed. Next, an oxide film 29 which is to be the oxide film19 a is formed (see FIG. 10A).

A material and a formation method which are similar to those of theoxide semiconductor film 18 in Embodiment 1 can be used for the oxidesemiconductor film 28. Further, the pair of electrodes 21 and 22 can beformed in a manner similar to that of FIG. 2B. Note that the pair ofelectrodes 21 and 22 is formed over the oxide semiconductor film 28. Amaterial and a formation method which are similar to those of the oxidefilm 19 in Embodiment 1 can be used for the oxide film 29.

Next, part of the oxide semiconductor film 28 and part of the oxide film29 are etched to form the multilayer film 20 including the oxidesemiconductor film 18 a and the oxide film 19 a (see FIG. 10B). Notethat the etching can be implemented with the use of a mask after themask is formed by a photolithography process over the oxide film whichis to be the oxide film 29. The oxide semiconductor film 28 and theoxide film 29 are concurrently etched; thus, the edge portion of theoxide semiconductor film 18 a is roughly aligned with the edge portionof the oxide film 19 a.

Next, the protective film 26 is formed to cover the gate insulating film17, the multilayer film 20, and the pair of electrodes 21 and 22 (seeFIG. 10C). The protective film 26 can be formed in a manner similar toEmbodiment 1. Further, in the method for manufacturing the transistor70, heat treatment can be performed with reference to Embodiment 1 asappropriate.

Further, by the etching for forming the pair of electrodes 21 and 22,defects such as oxygen vacancies are generated in the oxidesemiconductor film 18 a and the carrier density is increased in somecases; therefore, before the oxide film 29 is formed, the oxidesemiconductor film 18 a is preferably exposed to plasma generated in anoxygen atmosphere so that oxygen is supplied to the oxide semiconductorfilm 18 a. Thus, in the transistor 70, formation of a trap state in thevicinity of the interface between the oxide semiconductor film 18 a andthe oxide film 19 a can be inhibited, and change in threshold voltagecan be reduced. Further, in the transistor 70, leakage current thatflows in the vicinity of the side surface of the oxide semiconductorfilm 18 a in the multilayer film 20 can be reduced, and increase ofoff-state current can be inhibited.

Although the etching for forming the pair of electrodes 21 and 22damages the multilayer film 20 and generates oxygen vacancies on theback channel side of the multilayer film 20, part of oxygen contained inthe oxide insulating film 24 can be moved to the oxide semiconductorfilm 18 a, whereby the oxygen vacancies in the oxide semiconductor film18 a can be repaired. Accordingly, the reliability of the transistor 70can be improved.

Modification Example 1

In the transistor 70 described in this embodiment, the stacked-layerstructure of the multilayer film 20 and the pair of electrodes 21 and 22may be changed as appropriate. For example, a transistor as shown inFIG. 11 can be given as a modification example.

The transistor shown in FIG. 11 is different from the transistor 60 inthat an oxide semiconductor film 18 b and an oxide film 19 b are formedin different steps. That is, the edge portion of the oxide semiconductorfilm 18 b is covered with the pair of electrodes 21 and 22 and is not incontact with the oxide film 19 b.

The transistor shown in FIG. 11 is a transistor in which the contactresistance between the multilayer film 20 and the pair of electrodes 21and 22 is lower than that of the transistor 50 and the on-state currentis improved as compared to the transistor 50 because the pair ofelectrodes 21 and 22 is in direct contact with the oxide semiconductorfilm 18 b.

Further, since the pair of electrodes 21 and 22 is in direct contactwith the oxide semiconductor film 18 b in the transistor shown in FIG.11, the oxide film 19 b can be thickened without increase of the contactresistance between the multilayer film 20 and the pair of electrodes 21and 22. Thus, it is possible to inhibit formation of a trap state, whichoccurs due to plasma damage at the time of forming the protective film26, mixing of a constituent element of the protective film 26, or thelike, in the vicinity of the interface between the oxide semiconductorfilm 18 b and the oxide film 19 b. That is, the transistor shown in FIG.11 can achieve both improvement of on-state current and reduction ofchange in threshold voltage.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

Embodiment 4

In this embodiment, a transistor having a structure different from thoseof Embodiments 1 to 3 will be described with reference to FIG. 12. Atransistor 80 described in this embodiment includes a plurality of gateelectrodes facing each other with an oxide semiconductor film providedtherebetween.

The transistor 80 shown in FIG. 12 includes the gate electrode 15provided over the substrate 11. Moreover, the gate insulating film 17over the substrate 11 and the gate electrode 15, the oxide semiconductorfilm 18 overlapping with the gate electrode 15 with the gate insulatingfilm 17 provided therebetween, and the pair of electrodes 21 and 22 incontact with the oxide semiconductor film 18 are included. Furthermore,the protective film 26 including the oxide insulating film 23, the oxideinsulating film 24, and the nitride insulating film 25 is formed overthe gate insulating film 17, the oxide semiconductor film 18, and thepair of electrodes 21 and 22. Further, a gate electrode 61 overlappingwith the oxide semiconductor film 18 with the protective film 26provided therebetween is included.

The gate electrode 61 can be formed in a manner similar to that of thegate electrode 15.

The transistor 80 described in this embodiment has the gate electrode 15and the gate electrode 61 facing each other with the oxide semiconductorfilm 18 provided therebetween. By applying different potentials to thegate electrode 15 and the gate electrode 61, the threshold voltage ofthe transistor 80 can be controlled.

Further, when the oxide semiconductor film 18 in which the amount ofoxygen vacancies is reduced is included, the electrical characteristicsof the transistor can be improved. Further, the transistor in which theamount of change in threshold voltage is small and which is highlyreliable is obtained.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

Embodiment 5

In this embodiment, a transistor having a different structure from thetransistors in Embodiments 1 to 4 will be described with reference toFIGS. 13A to 13C.

In this embodiment, a semiconductor device having a transistor in whichthe amount of defects in an oxide semiconductor film can be furtherreduced as compared to Embodiments 1 to 4 is described with reference todrawings. The transistor described in this embodiment is different fromthose in Embodiments 1 to 4 in that the back channel side of the oxidesemiconductor film 18 is covered with the protective film and is notexposed to plasma generated in the etching treatment for forming thepair of electrodes.

FIGS. 13A to 13C are a top view and cross-sectional views of atransistor 90 included in the semiconductor device. FIG. 13A is a topview of the transistor 90, FIG. 13B is a cross-sectional view takenalong dashed-dotted line A-B in FIG. 13A, and FIG. 13C is across-sectional view taken along dashed-dotted line C-D in FIG. 13A.Note that in FIG. 13A, the substrate 11, one or more of components ofthe transistor 90 (e.g., the gate insulating layer 17), the oxideinsulating film 23, the oxide insulating film 24, the nitride insulatingfilm 25, and the like are not illustrated for clarity.

The transistor 90 shown in FIGS. 13A to 13C includes the gate electrode15 provided over the substrate 11. Moreover, the gate insulating film 17over the substrate 11 and the gate electrode 15, and the oxidesemiconductor film 18 overlapping with the gate electrode 15 with thegate insulating film 17 provided therebetween are provided. Further, theprotective film 26 including the oxide insulating film 23, the oxideinsulating film 24, and the nitride insulating film 25 is provided overthe gate insulating film 17 and the oxide semiconductor film 18, and apair of electrodes 21 b and 22 b which is formed over the protectivefilm 26 and is connected to the oxide semiconductor film 18 in theopening of the protective film 26 is provided.

Next, a method for manufacturing the transistor 90 is described.

In a manner similar to Embodiment 1, the gate electrode 15 is formedover the substrate 11, and the gate insulating film 17 is formed overthe substrate 11 and the gate electrode 15. Next, the oxidesemiconductor film 18 is formed over the gate insulating film 17.

Next, in a manner similar to Embodiment 1, after the oxide insulatingfilm 23 is formed over the gate insulating film 17 and the oxidesemiconductor film 18 while heating is performed at a temperature higherthan or equal to 280° C. and lower than or equal to 400° C., the oxideinsulating film 24 and the nitride insulating film 25 are formed. Notethat after the oxide insulating film 24 is formed, heat treatment isperformed to supply part of oxygen contained in the oxide insulatingfilm 24 to the oxide semiconductor film 18.

Next, parts of the oxide insulating film 23, the oxide insulating film24, and the nitride insulating film 25 are etched to form an openingwhich exposes part of the oxide semiconductor film 18. After that, thepair of electrodes 21 b and 22 b in contact with the oxide semiconductorfilm 18 is formed in a manner similar to Embodiment 1.

In this embodiment, the oxide semiconductor film 18 is covered with theprotective film 26 at the time of etching the pair of electrodes 21 band 22 b; thus, the oxide semiconductor film 18, particularly a backchannel region of the oxide semiconductor film 18, is not damaged by theetching for forming the pair of electrodes 21 b and 22 b. Further, theoxide insulating film 24 is formed using an oxide insulating film whichcontains oxygen at a higher proportion than the stoichiometriccomposition. Therefore, part of oxygen contained in the oxide insulatingfilm 24 can be moved to the oxide semiconductor film 18 to compensatethe oxygen vacancies in the oxide semiconductor film 18. Consequently,the amount of oxygen vacancies in the oxide semiconductor film 18 can bereduced.

By the above-described process, defects contained in the oxidesemiconductor film 18 can be reduced, and thus, the reliability of thetransistor 90 can be improved.

Embodiment 6

In this embodiment, a transistor having a different structure from thetransistors in Embodiments 1 to 5 will be described with reference toFIGS. 14A to 14C.

In this embodiment, a semiconductor device having a transistor in whichthe amount of defects in an oxide semiconductor film can be furtherreduced as compared to Embodiments 1 to 4 is described with reference todrawings. The transistor described in this embodiment is different fromthose in Embodiments 1 to 4 in that the back channel side of the oxidesemiconductor film 18 is covered with the protective film and is notexposed to plasma generated in the etching treatment for forming thepair of electrodes in a manner similar to Embodiment 5.

FIGS. 14A to 14C are a top view and cross-sectional views of atransistor 100 included in the semiconductor device. The transistor 100shown in FIGS. 14A to 14C is a channel protective type transistor. FIG.14A is a top view of the transistor 100, FIG. 14B is a cross-sectionalview taken along dashed-dotted line A-B in FIG. 14A, and FIG. 14C is across-sectional view taken along dashed-dotted line C-D in FIG. 14A.Note that in FIG. 14A, the substrate 11 and one or more of components ofthe transistor 100 (e.g., the gate insulating layer 17) are notillustrated for clarity.

The transistor 100 shown in FIGS. 14A to 14C includes the gate electrode15 over the substrate 11. Moreover, the gate insulating film 17 over thesubstrate 11 and the gate electrode 15, and the oxide semiconductor film18 overlapping with the gate electrode 15 with the gate insulating film17 provided therebetween are provided. Further, the protective film 26 aincluding an oxide insulating film 23 a, an oxide insulating film 24 a,and a nitride insulating film 25 a is provided over the gate insulatingfilm 17 and the oxide semiconductor film 18, and a pair of electrodes 21c and 22 c which is formed over the gate insulating film 17, the oxidesemiconductor film 18, and the protective film 26 a is provided.

Next, a method for manufacturing the transistor 100 is described.

In a manner similar to Embodiment 1, the gate electrode 15 is formedover the substrate 11, and the gate insulating film 17 is formed overthe substrate 11 and the gate electrode 15. Next, the oxidesemiconductor film 18 is formed over the gate insulating film 17.

Next, in a manner similar to Embodiment 1, after the oxide insulatingfilm 23 is formed over the gate insulating film 17 and the oxidesemiconductor film 18 while heating is performed at a temperature higherthan or equal to 280° C. and lower than or equal to 400° C., the oxideinsulating film 24 and the nitride insulating film 25 are formed. Notethat after the oxide insulating film 24 is formed, heat treatment isperformed to supply part of oxygen contained in the oxide insulatingfilm 24 to the oxide semiconductor film 18.

Next, parts of the oxide insulating film 23, the oxide insulating film24, and the nitride insulating film 25 are etched to form the protectivefilm 26 a including the oxide insulating film 23 a, the oxide insulatingfilm 24 a, and the nitride insulating film 25 a.

After that, the pair of electrodes 21 c and 22 c in contact with theoxide semiconductor film 18 is formed in a manner similar to Embodiment1.

In this embodiment, the oxide semiconductor film 18 is covered with theprotective film 26 a at the time of etching the pair of electrodes 21 cand 22 c; thus, the oxide semiconductor film 18 is not damaged by theetching for forming the pair of electrodes 21 c and 22 c. Further, theoxide insulating film 24 a is formed using an oxide insulating filmwhich contains oxygen at a higher proportion than the stoichiometriccomposition. Therefore, part of oxygen contained in the oxide insulatingfilm 24 a can be moved to the oxide semiconductor film 18 to compensatethe oxygen vacancies in the oxide semiconductor film 18. Consequently,the amount of oxygen vacancies in the oxide semiconductor film 18 can bereduced.

Note that the nitride insulating film 25 a is formed in the protectivefilm 26 a in FIGS. 14A to 14C; however, the protective film 26 a mayhave a stacked-layer structure of the oxide insulating film 23 a and theoxide insulating film 24 a. In that case, the nitride insulating film 25a is preferably formed after the pair of electrodes 21 c and 22 c isformed. Thus, hydrogen, water, or the like can be prevented fromentering the oxide semiconductor film 18 from the outside.

By the above-described process, defects contained in the oxidesemiconductor film 18 can be reduced, and thus, the reliability of thetransistor 100 can be improved.

Embodiment 7

Although the variety of films such as the metal film, the oxidesemiconductor film, and the inorganic insulating film which aredescribed in the above embodiments can be formed by a sputtering methodor a plasma chemical vapor deposition (CVD) method, such films may beformed by another method, e.g., a thermal CVD method. A metal organicchemical vapor deposition (MOCVD) method or an atomic layer deposition(ΔLD) method may be employed as an example of a thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to the chamber at a time,the pressure in a chamber is set to an atmospheric pressure or a reducedpressure, and reaction is caused in the vicinity of the substrate orover the substrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first gas so that thesource gases are not mixed, and then a second source gas is introduced.Note that in the case where the first source gas and the inert gas areintroduced at a time, the inert gas serves as a carrier gas, and theinert gas may also be introduced at the same time as the introduction ofthe second source gas. Alternatively, the first source gas may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second source gas may be introduced. The first sourcegas is adsorbed on the surface of the substrate to form a first layer;then the second source gas is introduced to react with the first layer;as a result, a second layer is stacked over the first layer, so that athin film is formed. The sequence of the gas introduction is repeatedplural times until a desired thickness is obtained, whereby a thin filmwith excellent step coverage can be formed. The thickness of the thinfilm can be adjusted by the number of repetitions times of the sequenceof the gas introduction; therefore, an ALD method makes it possible toaccurately adjust a thickness and thus is suitable for manufacturing aminute FET.

The variety of films such as the metal film, the oxide semiconductorfilm, and the inorganic insulating film which are described in the aboveembodiment can be formed by a thermal CVD method such as a MOCVD methodor an ALD method. For example, in the case where an In—Ga—Zn—O film isformed, trimethylindium, trimethylgallium, and dimethylzinc are used.Note that the chemical formula of trimethylindium is In(CH₃)₃. Thechemical formula of trimethylgallium is Ga(CH₃)₃. The chemical formulaof dimethylzinc is Zn(CH₃)₂. Without limitation to the abovecombination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be usedinstead of trimethylgallium and diethylzinc (chemical formula:Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed using adeposition apparatus employing ALD, two kinds of gases, i.e., ozone (O₃)as an oxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and a hafnium precursor compound (a hafniumalkoxide solution, typically tetrakis(dimethylamide)hafnium (TDMAH)) areused. Note that the chemical formula of tetrakis(dimethylamide)hafniumis Hf[N(CH₃)₂]₄. Examples of another material liquid includetetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed using adeposition apparatus employing ALD, two kinds of gases, e.g., H₂O as anoxidizer and a source gas which is obtained by vaporizing a solvent andliquid containing an aluminum precursor compound (e.g.,trimethylaluminum (TMA)) are used. Note that the chemical formula oftrimethylaluminum is Al(CH₃)₃. Examples of another material liquidinclude tris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed using adeposition apparatus employing ALD, hexachlorodisilane is adsorbed on asurface where a film is to be formed, chlorine contained in theadsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ ordinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed using adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced plural times to form an initial tungsten film,and then a WF₆ gas and an H₂ gas are introduced at a time, so that atungsten film is formed. Note that an SiH₄ gas may be used instead of aB₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is formed using a deposition apparatus employing ALD, anIn(CH₃)₃ gas and an O₃ gas are sequentially introduced plural times toform an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are introduced at atime to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas areintroduced at a time to form a ZnO layer. Note that the order of theselayers is not limited to this example. A mixed compound layer such as anIn—Ga—O layer, an In—Zn—O layer or a Ga—Zn—O layer may be formed bymixing of these gases. Note that although an H₂O gas which is obtainedby bubbling with an inert gas such as Ar may be used instead of an O₃gas, it is preferable to use an O₃ gas, which does not contain H.Further, instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used.Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Instead of anIn(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Furthermore, a Zn(CH₃)₂ gasmay be used.

Embodiment 8

In this embodiment, a semiconductor device of one embodiment of thepresent invention is described with reference to drawings. Note that inthis embodiment, a semiconductor device of one embodiment of the presentinvention is described taking a display device as an example.

FIG. 15A illustrates an example of a semiconductor device. Thesemiconductor device in FIG. 15A includes a pixel portion 101, a scanline driver circuit 104, a signal line driver circuit 106, m scan lines107 which are arranged in parallel or substantially in parallel andwhose potentials are controlled by the scan line driver circuit 104, andn signal lines 109 which are arranged in parallel or substantially inparallel and whose potentials are controlled by the signal line drivercircuit 106. Further, the pixel portion 101 includes a plurality ofpixels 301 arranged in a matrix. Furthermore, capacitor lines 115arranged in parallel or substantially in parallel are provided along thescan lines 107. Note that the capacitor lines 115 may be arranged inparallel or substantially in parallel along the signal lines 109. Thescan line driver circuit 104 and the signal line driver circuit 106 arecollectively referred to as a driver circuit portion in some cases.

Each scan line 107 is electrically connected to the n pixels 301 in thecorresponding row among the pixels 301 arranged in m rows and n columnsin the pixel portion 101. Each signal line 109 is electrically connectedto the m pixels 301 in the corresponding column among the pixels 301arranged in m rows and n columns. Note that m and n are each an integerof 1 or more. Each capacitor line 115 is electrically connected to the npixels 301 in the corresponding row among the pixels 301 arranged in mrows and n columns. Note that in the case where the capacitor lines 115are arranged in parallel or substantially in parallel along the signallines 109, each capacitor line 115 is electrically connected to the mpixels 301 in the corresponding column among the pixels 301 arranged inm rows and n columns.

FIGS. 15B and 15C illustrate circuit configurations that can be used forthe pixels 301 in the display device illustrated in FIG. 15A.

The pixel 301 illustrated in FIG. 15B includes a liquid crystal element132, a transistor 131_1, and a capacitor 133_1.

The potential of one of a pair of electrodes of the liquid crystalelement 132 is set according to the specifications of the pixels 301 asappropriate. The alignment state of the liquid crystal element 132depends on written data. A common potential may be applied to one of thepair of electrodes of the liquid crystal element 132 included in each ofthe plurality of pixels 301. Further, the potential supplied to one of apair of electrodes of the liquid crystal element 132 in the pixel 301 inone row may be different from the potential supplied to one of a pair ofelectrodes of the liquid crystal element 132 in the pixel 301 in anotherrow.

As examples of a driving method of the display device including theliquid crystal element 132, any of the following modes can be given: aTN mode, an STN mode, a VA mode, an ASM (axially symmetric alignedmicro-cell) mode, an OCB (optically compensated birefringence) mode, anFLC (ferroelectric liquid crystal) mode, an AFLC (antiferroelectricliquid crystal) mode, an MVA mode, a PVA (patterned vertical alignment)mode, an IPS mode, an FFS mode, a TBA (transverse bend alignment) mode,and the like. Other examples of the driving method of the display deviceinclude ECB (electrically controlled birefringence) mode, PDLC (polymerdispersed liquid crystal) mode, PNLC (polymer network liquid crystal)mode, and a guest-host mode. Note that the present invention is notlimited to these examples, and various liquid crystal elements anddriving methods can be applied to the liquid crystal element and thedriving method thereof.

The liquid crystal element may be formed using a liquid crystalcomposition including liquid crystal exhibiting a blue phase and achiral material. The liquid crystal exhibiting a blue phase has a shortresponse time of 1 msec or less and is optically isotropic; therefore,alignment treatment is not necessary and viewing angle dependence issmall.

In the pixel 301 in the m-th row and the n-th column, one of a sourceelectrode and a drain electrode of the transistor 131_1 is electricallyconnected to a signal line DL_n, and the other is electrically connectedto the other of a pair of electrodes of the liquid crystal element 132.A gate electrode of the transistor 131_1 is electrically connected to ascan line GL_m. The transistor 131_1 has a function of controllingwhether to write a data signal by being turned on or off.

One of a pair of electrodes of the capacitor 133_1 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a capacitor line CL), and the other is electricallyconnected to the other of the pair of electrodes of the liquid crystalelement 132. The potential of the capacitor line CL is set in accordancewith the specifications of the pixel 301 as appropriate. The capacitor133_1 functions as a storage capacitor for storing written data.

For example, in the display device including the pixel 301 in FIG. 15B,the pixels 301 are sequentially selected row by row by the scan linedriver circuit 104, whereby the transistors 131_1 are turned on and adata signal is written.

When the transistors 131_1 are turned off, the pixels 301 in which thedata has been written are brought into a holding state. This operationis sequentially performed row by row; thus, an image is displayed.

The pixel 301 illustrated in FIG. 15C includes a transistor 1312, acapacitor 1332, a transistor 134, and a light-emitting element 135.

One of a source electrode and a drain electrode of the transistor 1312is electrically connected to a wiring to which a data signal is supplied(hereinafter referred to as signal line DL_n). A gate electrode of thetransistor 131_2 is electrically connected to a wiring to which a gatesignal is supplied (hereinafter referred to as scan line GL_m).

The transistor 131_2 has a function of controlling whether to write adata signal by being turned on or off.

One of a pair of electrodes of the capacitor 133_2 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL_a), and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 131_2.

The capacitor 133_2 functions as a storage capacitor for storing writtendata.

One of a source electrode and a drain electrode of the transistor 134 iselectrically connected to the potential supply line VL_a. Further, agate electrode of the transistor 134 is electrically connected to theother of the source electrode and the drain electrode of the transistor131_2.

One of an anode and a cathode of the light-emitting element 135 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 134.

As the light-emitting element 135, an organic electroluminescent element(also referred to as an organic EL element) or the like can be used, forexample. Note that the light-emitting element 135 is not limited toorganic EL elements; an inorganic EL element including an inorganicmaterial can be used.

A high power supply potential VDD is supplied to one of the potentialsupply line VL_a and the potential supply line VL_b, and a low powersupply potential VSS is supplied to the other.

In the display device including the pixel 301 in FIG. 15C, the pixels301 are sequentially selected row by row by the scan line driver circuit104, whereby the transistors 131_2 are turned on and a data signal iswritten.

When the transistors 131_2 are turned off, the pixels 301 in which thedata has been written are brought into a holding state. Further, theamount of current flowing between the source electrode and the drainelectrode of the transistor 134 is controlled in accordance with thepotential of the written data signal. The light-emitting element 135emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage is displayed.

Note that in this specification and the like, a display element, adisplay device which is a device including a display element, alight-emitting element, and a light-emitting device which is a deviceincluding a light-emitting element can employ various modes or caninclude various elements. Examples of a display element, a displaydevice, a light-emitting element, or a light-emitting device include anEL (electroluminescent) element (e.g., an EL element including organicand inorganic materials, an organic EL element, or an inorganic ELelement), an LED (e.g., a white LED, a red LED, a green LED, or a blueLED), a transistor (a transistor which emits light depending oncurrent), an electron emitter, a liquid crystal element, electronic ink,an electrophoretic element, a grating light valve (GLV), a plasmadisplay panel (PDP), a micro electro mechanical system (MEMS), a digitalmicromirror device (DMD), a digital micro shutter (DMS), MIRASOL(registered trademark), an interferometric modulator display (IMOD), apiezoelectric ceramic display, or a carbon nanotube, which are displaymedia whose contrast, luminance, reflectivity, transmittance, or thelike is changed by electromagnetic action. Note that examples of adisplay device having an EL element include an EL display and the like.Examples of a display device having an electron emitter include a fieldemission display (FED), an SED-type flat panel display (SED:surface-conduction electron-emitter display), and the like. Examples ofa display device having a liquid crystal element include a liquidcrystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display) and the like. Examples of a display device having anelectronic ink or electrophoretic element include electronic paper.

Examples of an EL element are an element including an anode, a cathode,and an EL layer interposed between the anode and the cathode, and thelike. Examples of an EL layer include, but are not limited to, a layerutilizing light emission (fluorescence) from a singlet exciton, a layerutilizing light emission (phosphorescence) from a triplet exciton, alayer utilizing light emission (fluorescence) from a singlet exciton andlight emission (phosphorescence) from a triplet exciton, a layerincluding an organic material, a layer including an inorganic material,a layer including an organic material and an inorganic material, a layerincluding a high-molecular material, a layer including a low-molecularmaterial, a layer including a high-molecular material and alow-molecular material, and the like. Further, various types of ELelements can be used as well as these examples.

An example of liquid crystal elements is an element where transmissionand non-transmission of light is controlled by optical modulation actionof liquid crystals. The element can be configured to include a pair ofelectrodes and a liquid crystal layer. The optical modulation action ofliquid crystal is controlled by an electric field applied to the liquidcrystal (including a lateral electric field, a vertical electric fieldand a diagonal electric field). Note that specifically, the followingcan be used for a liquid crystal element: a nematic liquid crystal, acholesteric liquid crystal, a smectic liquid crystal, a discotic liquidcrystal, a thermotropic liquid crystal, a lyotropic liquid crystal, alow-molecular liquid crystal, a high-molecular liquid crystal, a polymerdispersed liquid crystal (PDLC), a ferroelectric liquid crystal, ananti-ferroelectric liquid crystal, a main-chain liquid crystal, aside-chain high-molecular liquid crystal, a banana-shaped liquidcrystal, and the like.

Next, a specific example of a liquid crystal display device including aliquid crystal element in the pixel 301 is described. FIG. 16 is a topview of the pixel 301 illustrated in FIG. 15B. Note that in FIG. 16, acounter electrode and a liquid crystal element are omitted.

In FIG. 16, a conductive film 304 c serving as a scan line extendssubstantially perpendicularly to the signal line (in the horizontaldirection in the drawing). A conductive film 310 d serving as a signalline extends substantially perpendicularly to the scan line (in thevertical direction in the drawing). A conductive film 310 f serving as acapacitor line extends in parallel to the signal line. Note that theconductive film 304 c serving as a scan line is electrically connectedto the scan line driver circuit 104 (see FIG. 15A), and the conductivefilm 310 d serving as a signal line and the conductive film 310 fserving as a capacitor line are electrically connected to the signalline driver circuit 106 (see FIG. 15A).

The transistor 103 is provided at a region where the scan line and thesignal line cross each other. The transistor 103 includes the conductivefilm 304 c serving as a gate electrode; the gate insulating film (notillustrated in FIG. 16); an oxide semiconductor film 308 b where achannel region is formed, over the gate insulating film; and theconductive films 310 d and 310 e serving as a source electrode and adrain electrode. The conductive film 304 c also serves as a scan line,and a region of the conductive film 304 c that overlaps with the oxidesemiconductor film 308 b serves as the gate electrode of the transistor103. In addition, the conductive film 310 d also serves as a signalline, and a region of the conductive film 310 d that overlaps with theoxide semiconductor film 308 b serves as the source electrode or drainelectrode of the transistor 103. Further, in the top view of FIG. 16, anend portion of the scan line is located on the outer side than an endportion of the oxide semiconductor film 308 b. Thus, the scan linefunctions as a light-blocking film for blocking light from a lightsource such as a backlight. For this reason, the oxide semiconductorfilm 308 b included in the transistor is not irradiated with light, sothat a variation in the electrical characteristics of the transistor canbe suppressed.

The conductive film 310 e is electrically connected to thelight-transmitting conductive film 316 b that serves as a pixelelectrode, through an opening 362 c.

The capacitor 105 is connected to the conductive film 310 f serving as acapacitor line through the opening 362. The capacitor 105 includes thelight-transmitting conductive film 308 c formed over the gate insulatingfilm, a dielectric film formed of a nitride insulating film formed overthe transistor 103, and a light-transmitting conductive film 316 b thatserves as a pixel electrode. That is, the capacitor 105 has alight-transmitting property.

Thanks to the light-transmitting property of the capacitor 105, thecapacitor 105 can be formed large (covers a large area) in the pixel301. Thus, a semiconductor device having charge capacity increased whileimproving the aperture ratio, to 50% or more, preferably 55% or more,more preferably 60% or more can be obtained. For example, in asemiconductor device with a high resolution such as a liquid crystaldisplay device, the area of a pixel is small and thus the area of acapacitor is also small. For this reason, the charge capacity of thecapacitor is small. However, since the capacitor 105 of this embodimenthas a light-transmitting property, when it is provided in a pixel,enough charge capacity can be obtained in the pixel and the apertureratio can be improved. Typically, the capacitor 105 can be favorablyused in a high-resolution semiconductor device with a pixel density of200 ppi or more, or furthermore, 300 ppi or more.

The pixel 301 illustrated in FIG. 16 has a shape in which a sideparallel to the conductive film 304 c serving as a scan line is longerthan a side parallel to the conductive film 310 d serving as a signalline and the conductive film 310 f serving as a capacitor line extendsin parallel to the conductive film 310 d serving as a signal line. As aresult, the area where the conductive film 310 f occupies the pixel 301can be decreased, thereby increasing the aperture ratio. In addition,the conductive film 310 f serving as a capacitor line does not use aconnection electrode, and is in a direct contact with thelight-transmitting conductive film 308 c and thus the aperture ratio canbe further increased.

Further, according to one embodiment of the present invention, theaperture ratio can be improved even in a display device with a highresolution, which makes it possible to use light from a light sourcesuch as a backlight efficiently, so that power consumption of thedisplay device can be reduced.

Next, FIG. 17 shows a cross section taken along dashed-dotted line C-Din FIG. 16. Note that a cross section A-B in FIG. 17 is across-sectional view of a driver circuit portion (a top view thereof isomitted) including the scan line driver circuit 104 and the signal linedriver circuit 106. In this embodiment, a liquid crystal display deviceof a vertical electric field mode is described.

In the liquid crystal display device described in this embodiment, aliquid crystal element 322 is provided between a pair of substrates (asubstrate 302 and a substrate 342).

The liquid crystal element 322 includes the light-transmittingconductive film 316 b over the substrate 302, films controllingalignment (hereinafter referred to as alignment films 318 and 352), aliquid crystal layer 320, and a conductive film 350. Note that thelight-transmitting conductive film 316 b functions as one electrode ofthe liquid crystal element 322, and the conductive film 350 functions asthe other electrode of the liquid crystal element 322.

Thus, a “liquid crystal display device” refers to a device including aliquid crystal element. Note that the liquid crystal display deviceincludes a driver circuit for driving a plurality of pixels and thelike. The liquid crystal display device may also be referred to as aliquid crystal module including a control circuit, a power supplycircuit, a signal generation circuit, a backlight module, and the likeprovided over another substrate.

In the driver circuit portion, the transistor 102 includes theconductive film 304 a functioning as a gate electrode, insulating films305 and 306 collectively functioning as a gate insulating film, theoxide semiconductor film 308 a in which a channel region is formed, andthe conductive films 310 a and 310 b functioning as a source electrodeand a drain electrode. The oxide semiconductor film 308 a is providedover the gate insulating film.

In the pixel portion, the transistor 103 includes the conductive film304 c functioning as a gate electrode, the insulating films 305 and 306collectively functioning as a gate insulating film, the oxidesemiconductor film 308 b which is formed over the gate insulating filmand in which a channel region is formed, and the conductive films 310 dand 310 e functioning as a source electrode and a drain electrode. Theoxide semiconductor film 308 b is provided over the gate insulatingfilm. Further, insulating films 312 and 314 are provided as protectivefilms over the conductive films 310 d and 310 e.

The light-transmitting conductive film 316 b functioning as a pixelelectrode is connected to the conductive film 310 e through an openingprovided in the insulating films 312 and 314.

Further, the capacitor 105 includes the light-transmitting conductivefilm 308 c functioning as one electrode of the capacitor 105, theinsulating film 314 functioning as a dielectric film, and thelight-transmitting conductive film 316 b functioning as the otherelectrode of the capacitor 105. The light-transmitting conductive film308 c is provided over the gate insulating film.

In the driver circuit portion, the conductive film 304 b formed at thesame time as the conductive films 304 a and 304 c and the conductivefilm 310 c formed at the same time as the conductive films 310 a, 310 b,310 d, and 310 e are connected to each other via the light-transmittingconductive film 316 a formed at the same time as the light-transmittingconductive film 316 b.

The conductive film 304 b and the light-transmitting conductive film 316a are connected to each other through an opening provided in theinsulating film 306 and the insulating film 312. Further, the conductivefilm 310 c and the light-transmitting conductive film 316 a areconnected to each other through an opening provided in the insulatingfilm 312 and the insulating film 314.

Here, components of the display device shown in FIG. 17 are describedbelow.

The conductive films 304 a, 304 b, and 304 c are formed over thesubstrate 302. The conductive film 304 a functions as a gate electrodeof the transistor in the driver circuit portion. The conductive film 304c is formed in the pixel portion 101 and functions as a gate electrodeof the transistor in the pixel portion. The conductive film 304 b isformed in the scan line driver circuit 104 and connected to theconductive film 310 c.

The substrate 302 can be formed using the material of the substrate 11which is given in Embodiment 1, as appropriate.

The conductive films 304 a, 304 b, and 304 c can be formed using thematerial and the formation method of the gate electrode 15 which aredescribed in Embodiment 1, as appropriate.

The insulating films 305 and 306 are formed over the substrate 302 andthe conductive films 304 a, 304 c, and 304 b. The insulating films 305and 306 function as a gate insulating film of the transistor in thedriver circuit portion and a gate insulating film of the transistor inthe pixel portion 101.

The insulating film 305 is preferably formed using the nitrideinsulating film which is described as the gate insulating film 17 inEmbodiment 1. The insulating film 306 is preferably formed using theoxide insulating film which is described as the gate insulating film 17in Embodiment 1.

The oxide semiconductor films 308 a and 308 b and the light-transmittingconductive film 308 c are formed over the insulating film 306. The oxidesemiconductor film 308 a is formed in a position overlapping with theconductive film 304 a and functions as a channel region of thetransistor in the driver circuit portion. The oxide semiconductor film308 b is formed in a position overlapping with the conductive film 304 cand functions as a channel region of the transistor in the pixelportion. The light-transmitting conductive film 308 c functions as oneelectrode of the capacitor 105.

The oxide semiconductor films 308 a and 308 b and the light-transmittingconductive film 308 c can be formed using the material and the formationmethod of the oxide semiconductor film 18 which are described inEmbodiment 1, as appropriate.

The light-transmitting conductive film 308 c is an oxide semiconductorfilm and contains impurities in a manner similar to those of the oxidesemiconductor films 308 a and 308 b. An example of the impurities ishydrogen. Instead of hydrogen, as the impurity, boron, phosphorus, tin,antimony, a rare gas element, alkali metal, alkaline earth metal, or thelike may be included.

Both the oxide semiconductor films 308 a and 308 b and thelight-transmitting conductive film 308 c are formed over the gateinsulating film but differ in impurity concentration. Specifically, thelight-transmitting conductive film 308 c has a higher impurityconcentration than the oxide semiconductor films 308 a and 308 b. Forexample, the concentration of hydrogen contained in each of the oxidesemiconductor films 308 a and 308 b is lower than 5×10¹⁹ atoms/cm³,preferably lower than 5×10¹⁸ atoms/cm³, more preferably lower than orequal to 1×10¹⁸ atoms/cm³, further preferably lower than or equal to5×10¹⁷ atoms/cm³, still further preferably lower than or equal to 1×10¹⁶atoms/cm³. The concentration of hydrogen contained in thelight-transmitting conductive film 308 c is higher than or equal to8×10¹⁹ atoms/cm³, preferably higher than or equal to 1×10²⁰ atoms/cm³,further preferably higher than or equal to 5×10²⁰ atoms/cm³. Theconcentration of hydrogen contained in the light-transmitting conductivefilm 308 c is greater than or equal to 2 times, preferably greater thanor equal to 10 times those in the oxide semiconductor films 308 a and308 b.

The light-transmitting conductive film 308 c has lower resistivity thanthe oxide semiconductor films 308 a and 308 b. The resistivity of thelight-transmitting conductive film 308 c is preferably greater than orequal to 1×10⁻⁸ times and less than or equal to 1×10⁻¹ times theresistivity of the oxide semiconductor films 308 a and 308 b. Theresistivity of the light-transmitting conductive film 308 c is typicallygreater than or equal to 1×10⁻³ Ωcm and less than 1×10⁴ Ωcm, preferablygreater than or equal to 1×10⁻³ Ωcm and less than 1×10⁻¹ Ωcm.

The oxide semiconductor films 308 a and 308 b are in contact with thefilms each formed using a material which can improve characteristics ofthe interface with the oxide semiconductor film, such as the insulatingfilm 306 and the insulating film 312. Thus, the oxide semiconductorfilms 308 a and 308 b function as semiconductors, so that thetransistors including the oxide semiconductor films 308 a and 308 b haveexcellent electrical characteristics.

The light-transmitting conductive film 308 c is in contact with theinsulating film 314 in the opening 362 (see FIG. 6A). The insulatingfilm 314 is formed using a material which prevents diffusion ofimpurities from the outside, such as water, alkali metal, and alkalineearth metal, into the oxide semiconductor film, and the material furtherincludes hydrogen. Thus, when hydrogen in the insulating film 314 isdiffused into the oxide semiconductor film formed at the same time asthe oxide semiconductor films 308 a and 308 b, hydrogen is bonded tooxygen and electrons serving as carriers are generated in the oxidesemiconductor film. Further, when the insulating film 314 is formed by aplasma CVD method or a sputtering method, the oxide semiconductor filmis exposed to plasma, so that oxygen vacancies are generated. Whenhydrogen contained in the insulating film 314 enters the oxygenvacancies, electrons serving as carriers are generated. As a result, theoxide semiconductor film has higher conductivity and functions as aconductor; in other words, the oxide semiconductor film can be an oxidesemiconductor film with high conductivity. Here, a metal oxide whichcontains a material similar to those of the oxide semiconductor films308 a and 308 b as a main component and has higher conductivity becausehydrogen concentration of the metal oxide is higher than those of theoxide semiconductor films 308 a and 308 b is referred to as the“light-transmitting conductive film 308 c”.

Note that one embodiment of the present invention is not limitedthereto, and it is possible that the light-transmitting conductive film308 c be not in contact with the insulating film 314 depending oncircumstances.

Further, one embodiment of the present invention is not limited thereto,and the light-transmitting conductive film 308 c may be formed by adifferent process from that of the oxide semiconductor film 308 a or theoxide semiconductor film 308 b depending on circumstances. In that case,the light-transmitting conductive film 308 c may include a differentmaterial from that of the oxide semiconductor film 308 a or the oxidesemiconductor film 308 b. For example, the light-transmitting conductivefilm 308 c may include indium tin oxide (hereinafter referred to asITO), indium zinc oxide, or the like.

In the semiconductor device illustrated in this embodiment, oneelectrode of the capacitor is formed at the same time as the oxidesemiconductor film of the transistor. In addition, thelight-transmitting conductive film that serves as a pixel electrode isused as the other electrode of the capacitor. Thus, a step of forminganother conductive film is not needed to form the capacitor, and thenumber of steps of manufacturing the semiconductor device can bereduced. Further, since the capacitor has a pair of electrodes formedwith the light-transmitting conductive film, it can have alight-transmitting property. As a result, the area occupied by thecapacitor can be increased and the aperture ratio in a pixel can beincreased.

The conductive films 310 a, 310 b, 310 c, 310 d, and 310 e can be formedusing the material and the formation method of the pair of electrodes 21and 22 which are described in Embodiment 1, as appropriate.

The insulating films 312 and 314 are formed over the insulating film306, the oxide semiconductor films 308 a and 308 b, thelight-transmitting conductive film 308 c, and the conductive films 310a, 310 b, 310 c, 310 d, and 310 e. For the insulating film 312, in amanner similar to that of the insulating film 306, a material which canimprove characteristics of the interface with the oxide semiconductorfilm is preferably used. The insulating film 312 can be formed using amaterial and a formation method which are similar to those of the oxideinsulating film 24 which are described in at least Embodiment 1, asappropriate. Further, as described in Embodiment 1, the oxide insulatingfilm 23 and the oxide insulating film may be stacked.

For the insulating film 314, in a manner similar to that of theinsulating film 305, a material which prevents diffusion of impuritiesfrom the outside, such as water, alkali metal, and alkaline earth metal,into the oxide semiconductor film is preferably used. The insulatingfilm 314 can be formed using the material and the formation method ofthe nitride insulating film 25 which are described in Embodiment 1, asappropriate.

Further, the light-transmitting conductive films 316 a and 316 b areprovided over the insulating film 314. The light-transmitting conductivefilm 316 a is electrically connected to the conductive film 304 bthrough the opening 364 a (see FIG. 20C) and electrically connected tothe conductive film 310 c through the opening 364 b (see FIG. 20C). Thatis, the light-transmitting conductive film 316 a functions as aconnection electrode which connects the conductive film 304 b and theconductive film 310 c. The light-transmitting conductive film 316 b iselectrically connected to the conductive film 310 e through the opening364 c (see FIG. 20C) and functions as the pixel electrode of a pixel.Further, the light-transmitting conductive film 316 b can function asone of the pair of electrodes of the capacitor.

In order to form a connection structure in which the conductive film 304b is in direct contact with the conductive film 310 c, it is necessaryto perform patterning for forming an opening in the insulating films 305and 306 and to form a mask before the conductive film 310 c is formed.However, the photomask is not needed to obtain the connection structurein FIG. 17. When the conductive film 304 b is connected to theconductive film 310 c with the light-transmitting conductive film 316 aas shown in FIG. 17, it is not necessary to form a connection portionwhere the conductive film 304 b is in direct contact with the conductivefilm 310 c. Thus, the number of photomasks can be reduced by one. Thatis, steps of forming a semiconductor device can be reduced.

For the light-transmitting conductive films 316 a and 316 b, alight-transmitting conductive material such as indium oxide includingtungsten oxide, indium zinc oxide including tungsten oxide, indium oxideincluding titanium oxide, indium tin oxide including titanium oxide,ITO, indium zinc oxide, or indium tin oxide to which silicon oxide isadded can be used.

A film having a colored property (hereinafter referred to as a coloredfilm 346) is formed on the substrate 342. The colored film 346 functionsas a color filter. Further, a light-blocking film 344 adjacent to thecolored film 346 is formed on the substrate 342. The light-blocking film344 functions as a black matrix. The colored film 346 is not necessarilyprovided in the case where the display device is a monochrome displaydevice, for example.

The colored film 346 is a colored film that transmits light in aspecific wavelength range. For example, a red (R) color filter fortransmitting light in a red wavelength range, a green (G) color filterfor transmitting light in a green wavelength range, a blue (B) colorfilter for transmitting light in a blue wavelength range, or the likecan be used.

The light-blocking film 344 preferably has a function of blocking lightin a particular wavelength region, and can be a metal film or an organicinsulating film including a black pigment.

An insulating film 348 is formed on the colored film 346. The insulatingfilm 348 functions as a planarization layer or suppresses diffusion ofimpurities in the colored film 346 to the liquid crystal element side.

The conductive film 350 is formed on the insulating film 348. Theconductive film 350 functions as the other of the pair of electrodes ofthe liquid crystal element in the pixel portion. Note that an insulatingfilm that functions as an alignment film may be additionally formed onthe light-transmitting conductive films 316 a and 316 b and theconductive film 350.

The liquid crystal layer 320 is formed between the light-transmittingconductive film 316 a and the conductive film 350, and thelight-transmitting conductive film 316 b and the conductive film 350.The liquid crystal layer 320 is sealed between the substrate 302 and thesubstrate 342 with the use of a sealant (not illustrated). The sealantis preferably in contact with an inorganic material to prevent entry ofmoisture and the like from the outside.

A spacer may be provided between the light-transmitting conductive film316 a and the conductive film 350, and the light-transmitting conductivefilm 316 b and the conductive film 350 to maintain the thickness of theliquid crystal layer 320 (also referred to as a cell gap).

A formation method of the element portion over the substrate 302 in thesemiconductor device illustrated in FIG. 17 is described with referenceto FIGS. 18A to 18C, FIGS. 19A to 19C, FIGS. 20A to 20C, and FIGS. 21Aand 21B.

First, the substrate 302 is prepared. Here, a glass substrate is used asthe substrate 302.

Then, a conductive film is formed over the substrate 302 and processedinto desired regions, so that the conductive films 304 a, 304 b, and 304c are formed. The conductive films 304 a, 304 b, and 304 c can be formedin such a manner that a mask is formed in the desired regions by firstpatterning and regions not covered with the mask are etched (see FIG.18A).

The conductive films 304 a, 304 b, and 304 c can be typically formed byan evaporation method, a CVD method, a sputtering method, a spin coatingmethod, or the like.

Next, the insulating film 305 is formed over the substrate 302 and theconductive films 304 a, 304 b, and 304 c, and then the insulating film306 is formed over the insulating film 305 (see FIG. 18A).

The insulating films 305 and 306 can be formed by a sputtering method, aCVD method, or the like. Note that it is preferable that the insulatingfilms 305 and 306 be formed in succession in a vacuum, in which caseentry of impurities is suppressed.

Next, an oxide semiconductor film 307 is formed over the insulating film306 (see FIG. 18B).

The oxide semiconductor film 307 can be formed by a sputtering method, acoating method, a pulsed laser deposition method, a laser ablationmethod, or the like.

Next, the oxide semiconductor film 307 is processed into desiredregions, so that the island-shaped oxide semiconductor films 308 a, 308b, and 308 d are formed. The oxide semiconductor films 308 a, 308 b, and308 d can be formed in such a manner that a mask is formed in thedesired regions by second patterning and regions not covered with themask are etched. For the etching, dry etching, wet etching, or acombination of both can be employed (see FIG. 18C).

Next, a conductive film 309 is formed over the insulating film 306 andthe oxide semiconductor films 308 a, 308 b, and 308 d (see FIG. 19A).

The conductive film 309 can be formed by a sputtering method, forexample.

Then, the conductive film 309 is processed into desired regions, so thatthe conductive films 310 a, 310 b, 310 c, 310 d, and 310 e are formed.The conductive films 310 a, 310 b, 310 c, 310 d, and 310 e can be formedin such a manner that a mask is formed in the desired regions by thirdpatterning and regions not covered with the mask are etched (see FIG.19B).

Next, an insulating film 311 is formed to cover the insulating film 306,the oxide semiconductor films 308 a, 308 b, and 308 d, and theconductive films 310 a, 310 b, 310 c, 310 d, and 310 e (see FIG. 19C).

The insulating film 311 can be formed with a stacked-layer structureunder conditions similar to those for the oxide insulating film 23 andthe oxide insulating film 24 in Embodiment 1. When the oxide insulatingfilm 23 is formed while heating is performed as described in Embodiment1, hydrogen, water, or the like in the oxide semiconductor films 308 a,308 b, and 308 d can be released; thus, highly purified oxidesemiconductor films can be formed.

Next, the insulating film 311 is processed into desired regions so thatthe insulating film 312 and the opening 362 are formed. The insulatingfilm 311 and the opening 362 can be formed in such a manner that a maskis formed in a desired region by fourth patterning and regions notcovered with the mask are etched (see FIG. 20A).

The opening 362 is formed so as to expose the surface of the oxidesemiconductor film 308 d. An example of a formation method of theopening 362 includes, but not limited to, a dry etching method.Alternatively, a wet etching method or a combination of dry etching andwet etching can be employed for formation of the opening 362.

After that, in a manner similar to Embodiment 1, heat treatment can beperformed to move part of oxygen in the insulating film 311 to the oxidesemiconductor films 308 a and 308 b and compensate oxygen vacanciesincluded in the oxide semiconductor films 308 a and 308 b. Consequently,the amount of oxygen vacancies in the oxide semiconductor films 308 aand 308 b can be reduced.

Next, an insulating film 313 is formed over the insulating film 312 andthe oxide semiconductor film 308 d (see FIG. 20B).

The insulating film 313 is preferably formed using a material that canprevent an external impurity such as oxygen, hydrogen, water, alkalimetal, or alkaline earth metal, from diffusing into the oxidesemiconductor film, more preferably formed using the material includinghydrogen, and typically an inorganic insulating material containingnitrogen, such as a nitride insulating film, can be used. The insulatingfilm 313 can be formed by a CVD method, for example.

The insulating film 314 is a film formed using a material that preventsdiffusion of impurities from the outside, such as water, alkali metal,and alkaline earth metal, into the oxide semiconductor film, and thematerial further includes hydrogen. Thus, when hydrogen in theinsulating film 314 is diffused into the oxide semiconductor film 308 d,hydrogen is bonded to oxygen and electrons serving as carriers aregenerated in the oxide semiconductor film 308 d. As a result, theconductivity of the oxide semiconductor film 308 d is increased, so thatthe oxide semiconductor film 308 d becomes the light-transmittingconductive film 308 c.

The silicon nitride film is preferably formed at a high temperature tohave an improved blocking property; for example, the silicon nitridefilm is preferably formed at a temperature in the range from thesubstrate temperature of 100° C. to 400° C., more preferably at atemperature in the range from 300° C. to 400° C. When the siliconnitride film is formed at a high temperature, a phenomenon in whichoxygen is released from the oxide semiconductor used for the oxidesemiconductor films 308 a and 308 b and the carrier density is increasedis caused in some cases; therefore, the upper limit of the temperatureis a temperature at which the phenomenon is not caused.

Then, the insulating film 313 is processed into desired regions so thatthe insulating film 314 and the openings 364 a, 364 b, and 364 c areformed. The insulating film 314 and the openings 364 a, 364 b, and 364 ccan be formed in such a manner that a mask is formed in a desired regionby fifth patterning and regions not covered by the mask are etched (seeFIG. 20C).

The opening 364 a is formed so as to expose a surface of the conductivefilm 304 b. The opening 364 b is formed so as to expose the conductivefilm 310 c. The opening 364 c is formed so as to expose the conductivefilm 310 e.

An example of a formation method of the openings 364 a, 364 b, and 364 cincludes, but not limited to, a dry etching method. Alternatively, a wetetching method or a combination of dry etching and wet etching can beemployed for formation of the openings 364 a, 364 b, and 364 c.

Then, a conductive film 315 is formed over the insulating film 314 so asto cover the openings 364 a, 364 b, and 364 c (see FIG. 21A).

The conductive film 315 can be formed by a sputtering method, forexample.

Then, the conductive film 315 is processed into desired regions so thatthe light-transmitting conductive films 316 a and 316 b are formed. Thelight-transmitting conductive films 316 a and 316 b are formed in such amanner that a mask is formed in the desired regions by sixth patterningand regions not covered with the mask are etched (see FIG. 21B).

Through the above process, the pixel portion and the driver circuitportion that include transistors can be formed over the substrate 302.In the manufacturing process described in this embodiment, thetransistors and the capacitor can be formed at the same time by thefirst to sixth patterning, that is, with the six masks.

In this embodiment, the conductivity of the oxide semiconductor film 308d is increased by diffusing hydrogen contained in the insulating film314 into the oxide semiconductor film 308 d; however, the conductivityof the oxide semiconductor film 308 d may be increased by covering theoxide semiconductor films 308 a and 308 b with a mask and addingimpurities, typically, hydrogen, boron, phosphorus, tin, antimony, arare gas element, alkali metal, alkaline earth metal, or the like to theoxide semiconductor film 308 d. Hydrogen, boron, phosphorus, tin,antimony, a rare gas element, or the like is added to the oxidesemiconductor film 308 d by an ion doping method, an ion implantationmethod, or the like. Further, alkali metal, alkaline earth metal, or thelike may be added to the oxide semiconductor film 308 d by a method inwhich the oxide semiconductor film 308 d is exposed to a solution thatcontains the impurity.

Next, a structure that is formed over the substrate 342 provided so asto face the substrate 302 is described below.

First, the substrate 342 is prepared. For materials of the substrate342, the materials that can be used for the substrate 302 can bereferred to. Then, the light-blocking film 344 and the colored film 346are formed over the substrate 342 (see FIG. 22A).

The light-blocking film 344 and the colored film 346 each are formed ina desired position with any of various materials by a printing method,an inkjet method, an etching method using a photolithography technique,or the like.

Then, the insulating film 348 is formed over the light-blocking film 344and the colored film 346 (see FIG. 22B).

For the insulating film 348, an organic insulating film of an acrylicresin, an epoxy resin, polyimide, or the like can be used. With theinsulating film 348, an impurity or the like contained in the coloredfilm 346 can be prevented from diffusing into the liquid crystal layer320, for example. Note that the insulating film 348 is not necessarilyformed.

Then, the conductive film 350 is formed over the insulating film 348(see FIG. 22C). As the conductive film 350, a material that can be usedfor the conductive film 315 can be used.

Through the above process, the structure formed over the substrate 342can be formed.

Next, the alignment film 318 and the alignment film 352 are formed overthe substrate 302 and the substrate 342 respectively, specifically, overthe insulating film 314 and the light-transmitting conductive films 316a and 316 b formed over the substrate 302 and over the conductive film350 formed over the substrate 342. The alignment films 318 and 352 canbe formed by a rubbing method, an optical alignment method, or the like.After that, the liquid crystal layer 320 is formed between the substrate302 and the substrate 342. The liquid crystal layer 320 can be formed bya dispenser method (a dropping method), or an injecting method by whicha liquid crystal is injected using a capillary phenomenon after thesubstrate 302 and the substrate 342 are bonded to each other.

Through the above process, the display device illustrated in FIG. 17 canbe fabricated.

This embodiment can be combined with another embodiment in thisspecification as appropriate.

Modification Example 1

A modification example of the liquid crystal display device including aliquid crystal element in the pixel 301 is described. FIG. 23 is a topview of the pixel 301 shown in FIG. 15B. Note that in FIG. 23, a counterelectrode and a liquid crystal element are omitted. Description of theportions similar to those in Embodiment 8 are omitted.

<Structure of Semiconductor Device>

The pixel 301 shown in FIG. 23 is different from the pixel 301 shown inFIG. 16 in that an opening 374 c is provided inside an opening 372 c.The pixel 301 shown in FIG. 23 is different from the pixel shown in FIG.17 in that an opening 372 is provided instead of the opening 364. Theconductive film 310 e is electrically connected to thelight-transmitting conductive film 316 b functioning as a pixelelectrode, through the opening 372 c and the opening 374 c.

Next, FIG. 24 shows a cross section taken along dashed-dotted line C-Din FIG. 23. Note that a cross section A-B in FIG. 24 is across-sectional view of a driver circuit portion (a top view thereof isomitted).

As shown in FIG. 24, an opening 372 a (see FIG. 25A) formed in theinsulating film 306 and the insulating film 312 and an opening 374 a(see FIG. 25C) formed in the insulating film 314 are provided over theconductive film 304 a. The opening 374 a (see FIG. 25C) is locatedinside the opening 372 a (see FIG. 25A). In the opening 374 a (see FIG.25C), the conductive film 304 a and the light-transmitting conductivefilm 316 a are connected to each other.

Further, an opening 372 b (see FIG. 25A) formed in the insulating film312 and an opening 374 b (see FIG. 25C) formed in the insulating film314 are provided over the conductive film 310 c. The opening 374 b (seeFIG. 25C) is located inside the opening 372 b (see FIG. 25A). In theopening 374 b (see FIG. 25C), the conductive film 310 c and thelight-transmitting conductive film 316 a are connected to each other.

Furthermore, the opening 372 c (see FIG. 25A) formed in the insulatingfilm 312 and the opening 374 c (see FIG. 25C) formed in the insulatingfilm 314 are provided over the conductive film 310 e. The opening 374 c(see FIG. 25C) is located inside the opening 372 c (see FIG. 25A). Inthe opening 374 c (see FIG. 25C), the conductive film 310 e and thelight-transmitting conductive film 316 b are connected to each other.

The opening 372 (see FIG. 25A) formed in the insulating film 312 isprovided over the light-transmitting conductive film 308 c. In theopening 372, the light-transmitting conductive film 308 c is in contactwith the insulating film 314.

A connection portion between the conductive film 304 b and thelight-transmitting conductive film 316 a, a connection portion betweenthe conductive film 310 c and the light-transmitting conductive film 316a, and a connection portion between the conductive film 310 e and thelight-transmitting conductive film 316 b are each surrounded by theinsulating film 305 and/or the insulating film 314. The insulating films305 and 314 are each formed of an insulating film using a material thatprevents diffusion of impurities from the outside, such as water, alkalimetal, and alkaline earth metal, into the oxide semiconductor film.Further, the side surfaces of the openings 372 a, 372 b, 372 c, and 372(see FIG. 25A) are each covered with the insulating film 305 and/or theinsulating film 314. The oxide semiconductor films are provided on aninner side than the insulating films 305 and 314. Thus, it is possibleto prevent diffusion of impurities from the outside, such as water,alkali metal, and alkaline earth metal, through the connection portionsbetween the conductive film 304 b and the light-transmitting conductivefilm 316 a, between the conductive film 310 c and the light-transmittingconductive film 316 a, between the conductive film 310 e and thelight-transmitting conductive film 316 b, and between thelight-transmitting conductive film 308 c and the light-transmittingconductive film 316 b into the oxide semiconductor films included in thetransistors. As a result, fluctuation in the electrical characteristicsof the transistors can be prevented and reliability of the semiconductordevice can be improved.

A formation method of the element portion over the substrate 302 in thesemiconductor device shown in FIG. 24 is described with reference toFIGS. 19A to 19C, FIGS. 25A to 25C, and FIGS. 26A and 26B.

In a manner similar to Embodiment 8, through the steps in FIGS. 18A to18C and FIGS. 19A to 19C, the conductive films 304 a, 304 b, and 304 c,each of which functions as a gate electrode, the insulating films 305and 306 which function as a gate insulating film, the oxidesemiconductor films 308 a, 308 b, and 308 d, the conductive films 310 a,310 b, 310 c, 310 d, and 310 e, and the insulating film 311 are formedover the substrate 302. Note that in the process, the first to thirdpatterning are performed to form the conductive films 304 a, 304 b, and304 c, the oxide semiconductor films 308 a 308 b, and 308 d, and theconductive films 310 a, 310 b, 310 c, 310 d, and 310 e.

Then, heat treatment is performed in a manner similar to Embodiment 8,whereby part of oxygen contained in the insulating film 311 can be movedto the oxide semiconductor films 308 a and 308 b to compensate theoxygen vacancies in the oxide semiconductor films 308 a and 308 b.Consequently, the amount of oxygen vacancies in the oxide semiconductorfilms 308 a and 308 b can be reduced.

Then, as shown in FIG. 25A, the insulating film 311 is processed intodesired regions so that the insulating film 312 and the openings 372,372 b, and 372 c are formed. Further, the insulating film 306 which ispart of the gate insulating film is processed into desired regions sothat the opening 372 a is formed. The insulating film 305, theinsulating film 312, and the openings 372, 372 a, 372 b, and 372 c canbe formed in such a manner that a mask is formed in the desired regionsby fourth patterning and regions not covered with the mask are etched.As a method for forming the openings 372, 372 a, 372 b, and 372 c, themethod for forming the opening 362 described in Embodiment 8 can be usedas appropriate.

By forming at least the opening 372 a in the etching step, the etchingamount can be reduced in an etching step with a mask formed by fifthpatterning to be performed later.

Next, the insulating film 313 is formed over the insulating film 305,the conductive films 310 c and 310 e, the insulating film 312, and theoxide semiconductor film 308 d (see FIG. 25B).

Then, in a manner similar to Embodiment 8, the insulating film 313 isprocessed into desired regions so that the insulating film 314 and theopenings 374 a, 374 b, and 374 c are formed. The insulating film 314 andthe openings 374 a, 374 b, and 374 c can be formed in such a manner thata mask is formed in the desired regions by fifth patterning and regionsnot covered with the mask are etched (see FIG. 25C).

Then, a conductive film 315 is formed over the insulating film 314 tocover the openings 374 a, 374 b, and 374 c (see FIG. 26A) in a mannersimilar to Embodiment 8.

Then, the conductive film 315 is processed into desired regions to formthe light-transmitting conductive films 316 a and 316 b. Thelight-transmitting conductive films 316 a and 316 b can be formed insuch a manner that a mask is formed in the desired regions by sixthpatterning and regions not covered with the mask are etched (see FIG.26B).

Through the above process, the pixel portion and the driver circuitportion that include transistors can be formed over the substrate 302.In the manufacturing process described in this embodiment, thetransistors and the capacitor can be formed at the same time by thefirst to sixth patterning, that is, with the six masks.

When the opening 372 a is not formed in the process of FIG. 25A, theinsulating films 305, 306, 312, and 314 are required to be etched in theetching step in FIG. 25C, so that the etching amount is increased ascompared to the case of forming other openings. Thus, the etching stepcannot be performed uniformly and the opening 374 a is not formed insome regions, so that a contact defect between the light-transmittingconductive film 316 a formed later and the conductive film 304 b isgenerated. However, in this embodiment, the openings 372 a and 374 a areformed in two etching steps; thus, an etching defect is not easilygenerated in the forming process of the openings. Consequently, yield ofa semiconductor device can be improved. The opening 372 a is describedhere, but the same effect can also obtained in the case of the openings374 b and 374 c.

Modification Example 2

A modification example of the liquid crystal display device including aliquid crystal element in the pixel 301 is described. In the liquidcrystal display devices shown in FIGS. 17 and 24, the light-transmittingconductive film 308 is in contact with the insulating film 314; however,a structure in which the light-transmitting conductive film 308 is incontact with the insulating film 305 can be employed. In that case, itis not necessary to provide the opening 362 as shown in FIGS. 20A to20C. Thus, unevenness of the surfaces of the light-transmittingconductive films 316 a and 316 b can be reduced. Consequently, alignmentdisorder of the liquid crystal materials contained in the liquid crystallayer 320 can be reduced. Further, a high-contrast semiconductor devicecan be fabricated.

Such a structure can be obtained as follows: in FIG. 18B, before theoxide semiconductor film 307 is formed, the insulating film 306 isselectively etched, so that part of the insulating film 305 is exposed.

Modification Example 3

A modification example of the semiconductor device described inEmbodiment 1 is described with reference to FIG. 27, FIGS. 28A to 28C,and FIGS. 29A to 29C. A cross section A-B in FIG. 27 is across-sectional view of a driver circuit portion, and a cross sectionC-D in FIG. 27 is a cross-sectional view of a pixel portion.

The semiconductor device shown in FIG. 27 is different from thesemiconductor device described in Embodiment 1 in that a channelprotective transistor is used.

In the driver circuit portion, the transistor 102 includes theconductive film 304 a functioning as a gate electrode, the insulatingfilms 305 and 306 collectively functioning as a gate insulating film,the oxide semiconductor film 308 a in which a channel region is formed,and the conductive films 310 a and 310 b functioning as a sourceelectrode and a drain electrode. The insulating film 312 functioning asa channel protective film is provided between the oxide semiconductorfilm 308 a and the conductive films 310 a and 310 b. Further, theinsulating film 314 is provided as a protective film over the conductivefilms 310 a, 310 b, and 310 c.

In the pixel portion, the transistor 103 includes the conductive film304 c functioning as a gate electrode, the insulating films 305 and 306collectively functioning as a gate insulating film, the oxidesemiconductor film 308 b which is formed over the gate insulating filmand in which a channel region is formed, and the conductive films 310 dand 310 e functioning as a source electrode and a drain electrode. Theinsulating film 312 functioning as a channel protective film is providedbetween the oxide semiconductor film 308 b and the conductive films 310d and 310 e. Further, the insulating film 314 is provided as aprotective film over the conductive films 310 d and 310 e and thelight-transmitting conductive film 308 c.

The light-transmitting conductive film 316 b functioning as a pixelelectrode is connected to the conductive film 310 e through an openingprovided in the insulating film 314.

Further, the capacitor 105 includes the light-transmitting conductivefilm 308 c functioning as one electrode of the capacitor 105, theinsulating film 314 functioning as a dielectric film, and thelight-transmitting conductive film 316 b functioning as the otherelectrode of the capacitor 105.

In the driver circuit portion, the conductive film 304 b formed at thesame time as the conductive films 304 a and 304 c and the conductivefilm 310 c formed at the same time as the conductive films 310 a, 310 b,310 d, and 310 e are connected to each other via the light-transmittingconductive film 316 a formed at the same time as the light-transmittingconductive film 316 b.

In this modification example, the oxide semiconductor films 308 a and308 b are not damaged by etching for forming the conductive films 310 a,310 b, 310 d, and 310 e because the oxide semiconductor films 308 a and308 b are covered with the insulating film 312 when the conductive films310 a, 310 b, 310 d, and 310 e are etched. Further, the insulating film312 is formed using an oxide insulating film which contains oxygen at ahigher proportion than the stoichiometric composition. Thus, part ofoxygen contained in the oxide insulating film 312 can be moved to theoxide semiconductor films 308 a and 308 b, so that the amount of oxygenvacancies contained in the oxide semiconductor films 308 a and 308 b canbe reduced.

A formation method of the element portion over the substrate 302 in thesemiconductor device shown in FIG. 27 is described with reference toFIGS. 19A to 19C, FIGS. 28A to 28C, and FIGS. 29A to 29C.

In a manner similar to Embodiment 8, through the steps in FIGS. 18A to18C, the conductive films 304 a, 304 b, and 304 c, each of whichfunctions as a gate electrode, the insulating films 305 and 306 whichfunction as a gate insulating film, and the oxide semiconductor films308 a, 308 b, and 308 d are formed over the substrate 302. Note that inthe process, the first and second patterning are performed to form theconductive films 304 a, 304 b, and 304 c and the oxide semiconductorfilms 308 a 308 b, and 308 d.

Next, as shown in FIG. 28A, the insulating film 311 is formed in amanner similar to Embodiment 8.

Then, heat treatment is performed in a manner similar to Embodiment 8,whereby part of oxygen contained in the insulating film 311 can be movedto the oxide semiconductor films 308 a and 308 b to compensate theoxygen vacancies in the oxide semiconductor films 308 a and 308 b.Consequently, the amount of oxygen vacancies in the oxide semiconductorfilms 308 a and 308 b can be reduced.

Next, as shown in FIG. 28B, the insulating film 311 is processed intodesired regions, so that the insulating film 312 is formed over theoxide semiconductor films 308 a and 308 b. In the process, in the casewhere the insulating film 306 is formed using a material similar to thatof the insulating film 312, part of the insulating film 306 is etched,and only regions covered with the oxide semiconductor films 308 a and308 b are left. Note that the insulating film 306 and the insulatingfilm 312 can be formed in such a manner that a mask is formed in thedesired regions by third patterning and regions not covered with themask are etched.

Next, after a conductive film is formed over the insulating film 305,the insulating film 306, and the oxide semiconductor films 308 a and 308b, a process similar to that in Embodiment 8 is performed, so that theconductive films 310 a, 310 b, 310 c, 310 d, and 310 e are formed (seeFIG. 28C). The conductive films 310 a, 310 b, 310 c, 310 d, and 310 ecan be formed in such a manner that a mask is formed in a desired regionby fourth patterning and regions not covered with the mask are etched.

Next, the insulating film 313 is formed over the insulating film 305,the insulating film 312, the oxide semiconductor film 308 d, and theconductive films 310 a, 310 b, 310 c, 310 d, and 310 e (see FIG. 29A).

Then, in a manner similar to Embodiment 8, the insulating film 313 isprocessed into desired regions, so that the insulating film 314 and theopenings 384 a, 384 b, and 384 c are formed. Note that the insulatingfilm 314 and the openings 384 a, 384 b, and 384 c can be formed in sucha manner that a mask is formed in the desired regions by fifthpatterning and regions not covered with the mask are etched (see FIG.29B).

Next, in a manner similar to Embodiment 8, after a conductive film isformed over the insulating film 314 to cover the openings 384 a, 384 b,and 384 c, the conductive film is processed into desired regions, sothat the light-transmitting conductive films 316 a and 316 b are formed(see FIG. 29C). The light-transmitting conductive films 316 a and 316 bcan be formed in such a manner that a mask is formed in the desiredregions by sixth patterning and regions not covered with the mask areetched.

Through the above process, the pixel portion and the driver circuitportion that include the transistors can be formed over the substrate302. In the manufacturing process described in this embodiment, thetransistors and the capacitor can be formed at the same time by thefirst to sixth patterning, that is, with the six masks.

Modification Example 4

In this embodiment and the modification example, the light-transmittingconductive film 308 c and the light-transmitting conductive film 316 bare used as the pair of electrodes for forming the capacitor 105.Instead of these films, as shown in FIG. 8, a light-transmittingconductive film 317 can be formed between the insulating film 312 andthe insulating film 314 and the light-transmitting conductive film 316 ccan be formed over the insulating film 314, so that thelight-transmitting conductive film 317 and the light-transmittingconductive film 316 c can be used as the pair of electrodes for formingthe capacitor 105.

Further, an organic insulating film of an acrylic resin, an epoxy resin,polyimide, or the like may be provided over the insulating film 312. Theorganic insulating film of an acrylic-based resin or the like can reduceunevenness of the surface of the light-transmitting conductive film 316a because of its high planarity. Thus, alignment disorder of the liquidcrystal materials contained in the liquid crystal layer 320 can bereduced. Further, a high-contrast semiconductor device can befabricated.

Modification Example 5

In this embodiment and the modification example, the light-transmittingconductive film 308 c and the light-transmitting conductive film 316 bare used as the pair of electrodes for forming the capacitor; however,two or more of the following can be selected as appropriate: aconductive film formed at the same time as the conductive films 304 a,304 b, and 304 c; a conductive film formed at the same time as theconductive films 310 a, 310 b, 310 c, 310 d, and 310 e; thelight-transmitting conductive film 308 c; and the light-transmittingconductive film 316 b.

Embodiment 9

In this embodiment, one embodiment which can be applied to the oxidesemiconductor film 18 and the multilayer films 20 and 34 in any of thetransistors included in the semiconductor device described in the aboveembodiment is described. Note that here, the oxide semiconductor filmincluded in the multilayer film is used as an example; further, theoxide film can have a similar structure.

The oxide semiconductor film may include one or more of the following:an oxide semiconductor having a single-crystal structure (hereinafterreferred to as a single-crystal oxide semiconductor); an oxidesemiconductor having a polycrystalline structure (hereinafter referredto as a polycrystalline oxide semiconductor); an oxide semiconductorhaving a microcrystalline structure (hereinafter referred to as amicrocrystalline oxide semiconductor), and an oxide semiconductor havingan amorphous structure (hereinafter referred to as an amorphous oxidesemiconductor). Further, the oxide semiconductor film may include aCAAC-OS. Furthermore, the oxide semiconductor film may include anamorphous oxide semiconductor and an oxide semiconductor having acrystal grain. Described below are the single-crystal oxidesemiconductor, the CAAC-OS, the polycrystalline oxide semiconductor, themicrocrystalline oxide semiconductor, and the amorphous oxidesemiconductor.

<Single Crystal Oxide Semiconductor>

The single crystal oxide semiconductor has, for example, a low impurityconcentration and a low density of defect states (a small amount ofoxygen vacancies), and thus has a low carrier density. Therefore, atransistor using the single crystal oxide semiconductor for a channelregion is unlikely to be normally on. Further, the single crystal oxidesemiconductor has a low density of defect states and thus has a lowdensity of trap states in some cases. Therefore, a transistor using thesingle crystal oxide semiconductor for a channel region has a smallvariation in electrical characteristics and high reliability in somecases.

<CAAC-OS>

The CAAC-OS film is one of oxide semiconductor films including aplurality of crystal parts, and most of the crystal parts each fitinside a cube whose one side is less than 100 nm Thus, there is a casewhere a crystal part included in the CAAC-OS film fits inside a cubewhose one side is less than 10 nm, less than 5 nm, or less than 3 nm.The density of defect states of the CAAC-OS film is lower than that ofthe microcrystalline oxide semiconductor film. The CAAC-OS film isdescribed in detail below.

In an image obtained with a transmission electron microscope (TEM), forexample, crystal parts can be found in the CAAC-OS in some cases. Inmost cases, in an image obtained with a TEM, crystal parts in theCAAC-OS each fit inside a cube whose one side is less than 100 nm, forexample. In an image obtained with a TEM, a boundary between the crystalparts in the CAAC-OS is not clearly observed in some cases. Further, inan image obtained with a TEM, a grain boundary in the CAAC-OS is notclearly observed in some cases. In the CAAC-OS, since a clear grainboundary does not exist, for example, segregation of an impurity isunlikely to occur. In the CAAC-OS, since a clear boundary does notexist, for example, high density of defect states is unlikely to occur.In the CAAC-OS, since a clear grain boundary does not exist, forexample, a reduction in electron mobility is unlikely to occur.

For example, the CAAC-OS includes a plurality of crystal parts. In theplurality of crystal parts, c-axes are aligned in a direction parallelto a normal vector of a surface where the CAAC-OS is formed or a normalvector of a surface of the CAAC-OS in some cases. When the CAAC-OS isanalyzed by an out-of-plane method with an X-ray diffraction (XRD)apparatus, a peak at 2θ of around 31 degrees which shows alignmentappears in some cases. Further, for example, spots (luminescent spots)are observed in an electron diffraction pattern of the CAAC-OS in somecases. An electron diffraction pattern obtained with an electron beamhaving a beam diameter of 10 nmφ or smaller, or 5 nmφ or smaller, iscalled a nanobeam electron diffraction pattern. In the CAAC-OS, forexample, among crystal parts, the directions of the a-axis and theb-axis of one crystal part are different from those of another crystalpart, in some cases. In the CAAC-OS, for example, c-axes are aligned,and a-axes and/or b-axes are not macroscopically aligned, in some cases.

FIG. 30 is an example of a nanobeam electron diffraction pattern of asample including a CAAC-OS. Here, the sample is cut in the directionperpendicular to a surface where the CAAC-OS is formed and the thicknessthereof is reduced to about 40 nm. Further, an electron beam with adiameter of 1 nmφ enters from the direction perpendicular to the cutsurface of the sample. FIG. 30 shows that spots are observed in thenanobeam electron diffraction pattern of the CAAC-OS.

In each of the crystal parts included in the CAAC-OS, for example, ac-axis is aligned in a direction parallel to a normal vector of asurface where the CAAC-OS is formed or a normal vector of a surface ofthe CAAC-OS, metal atoms are arranged in a triangular or hexagonalconfiguration when seen from the direction perpendicular to the a-bplane, and metal atoms are arranged in a layered manner or metal atomsand oxygen atoms are arranged in a layered manner when seen from thedirection perpendicular to the c-axis. Note that, among crystal parts,the directions of an a-axis and a b-axis of one crystal part may bedifferent from those of another crystal part. In this specification, aterm “perpendicular” includes a range from 80° to 100°, preferably from85° to 95°. In addition, a term “parallel” includes a range from −10° to10°, preferably from −5° to 5°.

Since the c-axes of the crystal parts included in the CAAC-OS arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS is formed or a normal vector of a surface of the CAAC-OS,the directions of the c-axes may be different from each other dependingon the shape of the CAAC-OS (the cross-sectional shape of the surfacewhere the CAAC-OS is formed or the cross-sectional shape of the surfaceof the CAAC-OS). Note that the film deposition is accompanied with theformation of the crystal parts or followed by the formation of thecrystal parts through crystallization treatment such as heat treatment.Hence, the c-axes of the crystal portions are aligned in the directionparallel to a normal vector of the surface where the CAAC-OS is formedor a normal vector of the surface of the CAAC-OS.

The CAAC-OS can be obtained by reducing the impurity concentration, forexample. The impurity herein means an element other than main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element such as silicon hasa higher strength to bond with oxygen than that of a metal elementincluded in the oxide semiconductor. Therefore, when the element takesoxygen away in the oxide semiconductor, the atomic arrangement in theoxide semiconductor is disrupted, whereby the crystallinity of the oxidesemiconductor is lowered in some cases. In addition, a heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disrupts the atomic arrangementin the oxide semiconductor, whereby the crystallinity of the oxidesemiconductor is lowered in some cases. Hence, the CAAC-OS is an oxidesemiconductor with a low impurity concentration. Note that the impurityincluded in the oxide semiconductor might serve as a carrier generationsource.

In the CAAC-OS, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS, in thecase where crystal growth occurs from a surface side of the oxidesemiconductor, the proportion of crystal parts in the vicinity of thesurface of the oxide semiconductor is higher than that in the vicinityof the surface where the oxide semiconductor is formed in some cases.Further, when an impurity is added to the CAAC-OS, the crystal part in aregion to which the impurity is added may have low crystallinity.

Further, the CAAC-OS can be formed, for example, by reducing the densityof defect states. In an oxide semiconductor, for example, oxygenvacancies cause an increase in the density of defect states. The oxygenvacancies serve as carrier traps or serve as carrier generation sourceswhen hydrogen is captured therein. In order to form the CAAC-OS, forexample, it is important to prevent oxygen vacancies from beinggenerated in the oxide semiconductor. Thus, the CAAC-OS is an oxidesemiconductor having a low density of defect states. In other words, theCAAC-OS is an oxide semiconductor having few oxygen vacancies.

Note that of the CAAC-OS, the absorption coefficient calculated by aconstant photocurrent method (CPM) is lower than 1×10⁻³/cm, preferablylower than 1×10⁻⁴/cm, further preferably lower than 5×10⁻⁵/cm. Theabsorption coefficient has a positive correlation with an energycorresponding to the localized levels due to oxygen vacancies and entryof impurities (the energy calculated from the wavelength); thus, thedensity of defect levels in the CAAC-OS is extremely low.

A part of the absorption coefficient which is called an urbach tail dueto the band tail is removed from a curve of the absorption coefficientobtained by the CPM measurement, whereby the absorption coefficient dueto the defect levels can be calculated from the following formula. Notethat the urbach tail indicates a constant gradient region on a curve ofthe absorption coefficient obtained by the CPM measurement, and thegradient is called urbach energy.

$\begin{matrix}{\int{\frac{{\alpha (E)} - \alpha_{u}}{E}{E}}} & \left\lbrack {{Formula}\mspace{20mu} 3} \right\rbrack\end{matrix}$

Here, α(E) indicates the absorption coefficient at each energy level andα_(u) indicates the absorption coefficient due to the urbach tail.

With the use of the highly purified intrinsic or substantially highlypurified intrinsic CAAC-OS in a transistor, variation in the electricalcharacteristics of the transistor due to irradiation with visible lightor ultraviolet light is small.

<Method for Forming CAAC-OS>

Since the c-axes of the crystal parts included in the CAAC-OS arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS is formed or a normal vector of a surface of the CAAC-OS,the directions of the c-axes may be different from each other dependingon the shape of the CAAC-OS (the cross-sectional shape of the surfacewhere the CAAC-OS is formed or the cross-sectional shape of the surfaceof the CAAC-OS). Note that when the CAAC-OS is formed, the direction ofc-axis of the crystal part is the direction parallel to a normal vectorof the surface where the CAAC-OS is formed or a normal vector of thesurface of the CAAC-OS. The crystal part is formed by film formation orby performing treatment for crystallization such as heat treatment afterfilm formation.

There are three methods for forming a CAAC-OS.

The first method is to form an oxide semiconductor film at a temperaturehigher than or equal to 100° C. and lower than or equal to 450° C. toform, in the oxide semiconductor film, crystal parts in which the c-axesare aligned in the direction parallel to a normal vector of a surfacewhere the oxide semiconductor film is formed or a normal vector of asurface of the oxide semiconductor film. Note that in thisspecification, the film formation temperature is preferably higher thanor equal to 100° C. and lower than or equal to 400° C.

The second method is to form an oxide semiconductor film with a smallthickness and then heat it at a temperature higher than or equal to 200°C. and lower than or equal to 700° C. to form, in the oxidesemiconductor film, crystal parts in which the c-axes are aligned in thedirection parallel to a normal vector of a surface where the oxidesemiconductor film is formed or to a normal vector of a surface of theoxide semiconductor film. Note that in this specification, the heatingtemperature is preferably higher than or equal to 200° C. and lower thanor equal to 400° C.

The third method is to form a first oxide semiconductor film with asmall thickness, then heat it at a temperature higher than or equal to200° C. and lower than or equal to 700° C., and form a second oxidesemiconductor film to form, in the second oxide semiconductor film,crystal parts in which the c-axes are aligned in the direction parallelto a normal vector of a surface where the second oxide semiconductorfilm is formed or to a normal vector of a surface of the second oxidesemiconductor film. Note that in this specification, the heatingtemperature is preferably higher than or equal to 200° C. and lower thanor equal to 400° C.

Here, the first method for forming a CAAC-OS is described.

<Target and Formation Method Thereof>

The CAAC-OS is formed by a sputtering method with a polycrystallineoxide semiconductor sputtering target. When ions collide with thesputtering target, a crystal region included in the sputtering targetmay be separated from the target along an a-b plane; in other words, asputtered particle having a plane parallel to an a-b plane(flat-plate-like sputtered particle or pellet-like sputtered particle)may flake off from the sputtering target. In that case, theflat-plate-like sputtered particle or the pellet-like sputtered particlereaches a surface on which the CAAC-OS is formed while maintaining itscrystal state, whereby the CAAC-OS can be deposited.

For the deposition of the CAAC-OS, the following conditions arepreferably used.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, and nitrogen) which exist in thedeposition chamber may be reduced. Furthermore, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas with a dew point of −80° C. or lower, preferably −100° C.or lower is used.

By increasing the heating temperature of the surface on which theCAAC-OS is formed (e.g., the substrate heating temperature) during thedeposition, migration of a sputtered particle is likely to occur afterthe sputtered particle reaches the surface on which the CAAC-OS isformed. Specifically, the temperature of the surface on which theCAAC-OS is formed during the deposition is higher than or equal to 100°C. and lower than or equal to 740° C., preferably higher than or equalto 200° C. and lower than or equal to 500° C. By increasing thetemperature of the surface on which the CAAC-OS is formed during thedeposition, when the flat-plate-like sputtered particle reaches thesurface on which the CAAC-OS is formed, migration occurs on the surfaceon which the CAAC-OS is formed, so that a flat plane of the sputteredparticle is attached to the surface on which the CAAC-OS is formed. Thediameter (equivalent circle diameter) of the plane of the sputteredparticle, which is parallel to the a-b plane, is approximately greaterthan or equal to 1 nm and less than or equal to 30 nm or greater than orequal to 1 nm and less than or equal to 10 nm, though it differsdepending on the kind of oxide. Note that the flat-plate-like sputteredparticle may have a hexagonal cylinder shape whose hexagonal plane isparallel to the a-b plane. In such a case, a direction perpendicular tothe hexagonal plane is a c-axis direction.

When a cation of oxygen is ejected to a sputtering target in thesputtering, it is possible to reduce plasma damage at the deposition.Thus, when the ion collides with the surface of the sputtering target, alowering in crystallinity of the sputtering target can be suppressed ora change of the sputtering target into an amorphous state can besuppressed.

When a cation of oxygen or argon is ejected to a sputtering target inthe sputtering, in the case where a flat-plate-like sputtered particlehaving a hexagonal columnar shape is sputtered, the corners of ahexagonal plane can be positively charged. When the corners of thehexagonal plane are positively charged, positive charges repel eachother in one sputtered particle. Thus, flat-plate shapes of thesputtered particles can be maintained.

It is preferable to use a direct-current (DC) power source to positivelycharge the corners of the plane of the flat-plate-like sputteredparticle. Note that a radio frequency (RF) power source or analternating-current (ΔC) power source can be used. Note that it isdifficult to use an RF power source for a sputtering apparatus which iscapable of deposition to a large-sized substrate. In addition, a DCpower source is preferred to an AC power source from the viewpointbelow.

In the AC power source, adjacent targets alternately have a cathodepotential and an anode potential. In the case where the flat-plate-likesputtered particle is positively charged, positive charges in thesputtered particle repel each other, whereby flat-plate shapes of thesputtered particles can be maintained. However, in the case where the ACpower source is used, there is time during which an electric field isnot applied instantaneously; therefore, some charges of theflat-plate-like sputtered particle are lost and the structure of thesputtered particle might be broken. Thus, a DC power source is preferredto an AC power source.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is 30 vol % or higher, preferably 100 vol %.

As an example of the sputtering target, an In—Ga—Zn-based compoundtarget is described below.

The polycrystalline In—Ga—Zn-based compound target is made by mixingInO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in a predeterminedmolar ratio, applying pressure, and performing heat treatment at atemperature higher than or equal to 1000° C. and lower than or equal to1500° C. This pressure treatment may be performed while cooling isperformed or may be performed while heating is performed. Note that X,Y, and Z are each a given positive number. Here, the predetermined molarratio of InO_(X) powder to GaO_(Y) powder and ZnO_(Z) powder is, forexample, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, 3:1:2, 1:3:2, 1:6:4, or1:9:6. The kinds of powder and the molar ratio for mixing powder may bedetermined as appropriate depending on the desired sputtering target.

With use of the sputtering target in the way as described above, anoxide semiconductor film having a uniform thickness and a uniformcrystal orientation can be formed.

<Polycrystalline Oxide Semiconductor>

Note that an oxide semiconductor including polycrystal is referred to asa polycrystalline oxide semiconductor. A polycrystalline oxidesemiconductor includes a plurality of crystal grains.

In an image obtained with a TEM, for example, crystal grains can befound in the polycrystalline oxide semiconductor in some cases. In mostcases, the size of a crystal grain in the polycrystalline oxidesemiconductor is greater than or equal to 2 nm and less than or equal to300 nm, greater than or equal to 3 nm and less than or equal to 100 nm,or greater than or equal to 5 nm and less than or equal to 50 nm in animage obtained with the TEM, for example. Moreover, in the TEM image, aboundary between crystal grains can be found in the polycrystallineoxide semiconductor in some cases. Moreover, in the TEM image, a grainboundary can be found in the polycrystalline oxide semiconductor in somecases.

The polycrystalline oxide semiconductor may include a plurality ofcrystal grains, and the alignment of crystals may be different in theplurality of crystal grains. When a polycrystalline oxide semiconductoris analyzed by an out-of-plane method with use of an XRD apparatus, apeak at 2θ of around 31 degrees which shows alignment or peaks showingplural kinds of alignment appear in some cases. Further, spots areobserved in a nanobeam electron diffraction pattern of thepolycrystalline oxide semiconductor in some cases.

The polycrystalline oxide semiconductor has high crystallinity and thushas high electron mobility in some cases. Accordingly, a transistorincluding the polycrystalline oxide semiconductor in a channel regionhas high field-effect mobility. Note that there are cases in which animpurity is segregated at the grain boundary between the crystals in thepolycrystalline oxide semiconductor. Moreover, the grain boundary of thepolycrystalline oxide semiconductor becomes a defect state. Since thegrain boundary of the polycrystalline oxide semiconductor may serve as acarrier trap or a carrier generation source, a transistor using thepolycrystalline oxide semiconductor for a channel region has largervariation in electrical characteristics and lower reliability than atransistor using a CAAC-OS for a channel region in some cases.

The polycrystalline oxide semiconductor can be formed byhigh-temperature heat treatment or laser light treatment.

<Microcrystalline Oxide Semiconductor>

In an image obtained with the TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor in some cases. In mostcases, the size of a crystal part in a microcrystalline oxidesemiconductor film is greater than or equal to 1 nm and less than orequal to 100 nm, or greater than or equal to 1 nm and less than or equalto 10 nm. A microcrystal with a size greater than or equal to 1 nm andless than or equal to 10 nm, or a size greater than or equal to 1 nm andless than or equal to 3 nm is specifically referred to as nanocrystal(nc). An oxide semiconductor film including nanocrystal is referred toas an nc-OS (nanocrystalline oxide semiconductor) film. In an imageobtained with TEM, a grain boundary cannot be found clearly in the nc-OSfilm in some cases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. Further, there is noregularity of crystal orientation between different crystal parts in thenc-OS film; thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than the diameter of a crystal part, a peak which showsa crystal plane does not appear. Further, a halo pattern is shown in aselected-area electron diffraction pattern of the nc-OS film obtained byusing an electron beam having a probe diameter larger than the diameterof a crystal part (e.g., larger than or equal to 50 nm). Meanwhile,spots are shown in a nanobeam electron diffraction pattern of the nc-OSfilm obtained by using an electron beam having a probe diameter (e.g.,larger than or equal to 1 nm and smaller than or equal to 30 nm) closeto, or smaller than or equal to that of a crystal part. Further, in ananobeam electron diffraction pattern of the nc-OS film, regions withhigh luminance in a circular (ring) pattern are shown in some cases.Also in a nanobeam electron diffraction pattern of the nc-OS film, aplurality of spots are shown in a ring-like region in some cases.

FIG. 31 shows an example of nanobeam electron diffraction performed on asample including an nc-OS film. The measurement position is changed.Here, the sample is cut in the direction perpendicular to a surfacewhere an nc-OS film is formed and the thickness thereof is reduced to beless than or equal to 10 nm. Further, an electron beam with a diameterof 1 nm enters from the direction perpendicular to the cut surface ofthe sample. FIG. 31 shows that, when a nanobeam electron diffraction isperformed on the sample including the nc-OS film, a diffraction patternexhibiting a crystal plane is obtained, but orientation along a crystalplane in a particular direction is not observed.

Since the nc-OS film is an oxide semiconductor film having moreregularity than the amorphous oxide semiconductor film, the nc-OS filmhas a lower density of defect states than the amorphous oxidesemiconductor film. However, there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Thus, the nc-OS film has a higher carrier density than the CAAC-OS filmin some cases. The oxide semiconductor film having a high carrierdensity has high electron mobility in some cases. Thus, a transistorincluding the nc-OS film has high field-effect mobility in some cases.The nc-OS film has a higher defect state density than the CAAC-OS film,and thus has a lot of carrier traps in some cases. Consequently, atransistor including the nc-OS film has larger variation in electriccharacteristics and lower reliability than a transistor including theCAAC-OS film. The nc-OS film can be formed easily as compared to theCAAC-OS film because the nc-OS film can be formed even when a relativelylarge amount of impurities are included; thus, depending on the purpose,the nc-OS film can be favorably used in some cases. Therefore, asemiconductor device including the transistor including the nc-OS filmcan be manufactured with high productivity in some cases.

<Method of Forming Microcrystalline Oxide Semiconductor Film>

Next, a method of forming the microcrystalline oxide semiconductor filmis described below. The microcrystalline oxide semiconductor film isformed by a sputtering method in an atmosphere containing oxygen at atemperature of higher than or equal to a room temperature and lower thanor equal to 75° C., preferably higher than or equal to a roomtemperature and lower than or equal to 50° C. With the use of theatmosphere containing oxygen, oxygen vacancies in the microcrystallineoxide semiconductor film can be reduced and a film including amicrocrystal part can be formed.

A reduction of oxygen vacancies in the microcrystalline oxidesemiconductor film allows the formation of a film having stable physicalproperties. In particular, in the case where a semiconductor device ismanufactured with the use of a microcrystalline oxide semiconductorfilm, oxygen vacancies in the microcrystalline oxide semiconductor filmserve as donors, and electrons that are carriers are generated in themicrocrystalline oxide semiconductor film, which causes change inelectrical characteristics of the semiconductor device. Thus, asemiconductor device formed using a microcrystalline oxide semiconductorfilm in which oxygen vacancies are reduced can be highly reliable.

Note that it is preferable to increase the oxygen partial pressure inthe deposition atmosphere because the oxygen vacancies in themicrocrystalline oxide semiconductor film can be further reduced. Morespecifically, the oxygen partial pressure in the deposition atmosphereis preferably greater than or equal to 33%.

Note that for a target used in formation of a microcrystalline oxidesemiconductor film by a sputtering method, a target and a forming methodwhich are similar to those of the CAAC-OS can be used.

Note that the nc-OS can be formed easily as compared to the CAAC-OSbecause the nc-OS can be formed even when a relatively large amount ofimpurities are included; thus, depending on the purpose, the nc-OS canbe favorably used in some cases. For example, the nc-OS may be formed bya deposition method such as a sputtering method using an AC powersupply. The sputtering method using an AC power supply allows a film tobe formed with high uniformity over a large substrate, so that asemiconductor device including a transistor using the nc-OS for achannel region can be manufactured with high productivity.

<Amorphous Oxide Semiconductor>

An amorphous oxide semiconductor, for example, has disordered atomicarrangement and no crystal part. An amorphous oxide semiconductor, forexample, does not have a specific shape as in quartz and regularity inatomic arrangement.

In an image obtained with a TEM, for example, crystal parts cannot befound clearly in the amorphous oxide semiconductor film in some cases.

When an amorphous oxide semiconductor is analyzed by an out-of-planemethod with an XRD apparatus, a peak which shows alignment does notappear in some cases. Further, a halo pattern is observed in an electrondiffraction pattern of an amorphous oxide semiconductor in some cases.In other cases, a halo pattern is observed instead of a spot in ananobeam electron diffraction pattern of the amorphous oxidesemiconductor.

The amorphous oxide semiconductor can be formed in some cases, forexample, by introducing a high-concentration impurity such as hydrogen.Thus, the amorphous oxide semiconductor contains impurities at a highconcentration.

When an oxide semiconductor contains a high-concentration impurity, adefect state such as an oxygen vacancy is formed in the oxidesemiconductor in some cases. This means that an amorphous oxidesemiconductor with a high-concentration impurity has a high density ofdefect states. In addition, since the amorphous oxide semiconductor haslow crystallinity, the density of defect states of the amorphous oxidesemiconductor is higher than that of the CAAC-OS or the nc-OS.

Accordingly, the amorphous oxide semiconductor has much higher carrierdensity than the nc-OS. Therefore, a transistor including the amorphousoxide semiconductor for a channel region tends to be normally on. Thus,in some cases, such an amorphous oxide semiconductor can be applied to atransistor which needs to be normally on. Since the amorphous oxidesemiconductor has a high density of defect states, density of carriertraps might be increased. Consequently, a transistor including theamorphous oxide semiconductor for a channel region has larger variationin electric characteristics and lower reliability than a transistorincluding the CAAC-OS or the nc-OS for a channel region. Note that theamorphous oxide semiconductor can be formed by a deposition method inwhich a relatively large amount of impurity is contained, and thus canbe easily obtained and preferably used depending on the application. Forexample, the amorphous oxide semiconductor may be formed by a depositionmethod such as a spin coating method, a sol-gel method, an immersionmethod, a spray method, a screen printing method, a contact printingmethod, an ink-jet printing method, a roll coating method, or a mist CVDmethod. Hence, a semiconductor device including a transistor using theamorphous oxide semiconductor for a channel region can be manufacturedwith high productivity.

Note that when the oxide semiconductor has few defects, the densitythereof is increased. When the oxide semiconductor has highcrystallinity, the density thereof is increased. When the oxidesemiconductor has a lower concentration of impurities such as hydrogen,the density thereof is increased. The single-crystal oxide semiconductorhas higher density than the CAAC-OS in some cases. The CAAC-OS hashigher density than the microcrystalline oxide semiconductor in somecases. The polycrystalline oxide semiconductor has higher density thanthe microcrystalline oxide semiconductor in some cases. Themicrocrystalline oxide semiconductor has higher density than theamorphous oxide semiconductor in some cases.

Embodiment 10

In this embodiment, a human interface to which the semiconductor deviceof one embodiment of the present invention can be applied is described.In particular, a structure example of a sensor that can detect proximityor touch of an object (hereinafter referred to as a touch sensor) isdescribed.

For a touch sensor, a variety of types such as a capacitive type, aresistive type, a surface acoustic wave type, and an infrared type canbe employed.

Examples of the capacitive touch sensor are typically of a surfacecapacitive type, a projected capacitive type, and the like. Further,examples of the projected capacitive type are of a self capacitive type,a mutual capacitive type, and the like mainly in accordance with thedifference in the driving method. Here, the use of a mutual capacitivetype is preferable because of simultaneous sensing of multiple points(also referred to as multipoint sensing or multi-touch).

Besides the touch sensor described here in detail, a sensor that candetect the operation (gesture) of an object (e.g., a finger or a hand),eye movements of users, or the like by a camera (including an infraredcamera) or the like can be used as a human interface.

<Example of Detection Method of Sensor>

FIGS. 32A and 32B are schematic diagrams each illustrating a structureof a mutual capacitive touch sensor and input and output waveforms. Thetouch sensor includes a pair of electrodes. Capacitance is formedbetween the pair of electrodes. Input voltage is input to one of thepair of electrodes. Further, a detection circuit which detects currentflowing in the other electrode (or a potential of the other electrode)is provided.

For example, in the case where a rectangular wave is used as an inputvoltage waveform as illustrated in FIG. 32A, a waveform having a sharppeak is detected as an output current waveform.

Further, in the case where an object having conductivity is proximate toor touches a capacitor as illustrated in FIG. 32B, the capacitance valuebetween the electrodes is decreased; accordingly, the current value ofthe output is decreased.

By detecting a change in capacitance by using a change in output current(or potential) with respect to input voltage in this manner, proximityor a touch of an object can be detected.

<Structure Example of Touch Sensor>

FIG. 32C illustrates a structure example of a touch sensor provided witha plurality of capacitors arranged in a matrix.

The touch sensor includes a plurality of wirings extending in an Xdirection (the horizontal direction of this figure) and a plurality ofwirings extending in a Y direction (the vertical direction of thisfigure) which intersect with the plurality of wirings. Capacitance isformed between two wirings intersecting with each other.

One of input voltage and a common potential (including a groundedpotential and a reference potential) is input to each of the wiringsextending in the X direction. Further, a detection circuit (e.g., asource meter or a sense amplifier) is electrically connected to thewirings extending in the Y direction and can detect current (orpotential) flowing through the wirings.

The touch sensor can perform sensing two dimensionally in such a mannerthat the touch sensor sequentially scans the plurality of wiringsextending in the X direction so that input voltage is input and detectsa change in current (or potential) flowing through the wirings extendingin the Y direction.

<Structure Example of Touchscreen>

A structure example of a touchscreen including a touch sensor and adisplay portion including a plurality of pixels and a case where thetouchscreen is incorporated in an electronic device are described below.

FIG. 33A is a schematic cross-sectional view of an electronic deviceincluding a touchscreen.

An electronic device 3530 includes a housing 3531 and at least atouchscreen 3532, a battery 3533, and a control portion 3534, which areprovided in the housing 3531. The touchscreen 3532 is electricallyconnected to the control portion 3534 through a wiring 3535. The controlportion 3534 controls image display on a display portion and the sensingoperation of the touch sensor. The battery 3533 is electricallyconnected to the control portion 3534 through a wiring 3536 to supplyelectric power to the control portion 3534.

The touchscreen 3532 is provided so that its surface is not covered. Animage can be displayed on the exposed surface of the touchscreen 3532and the proximity or the contact of an object can be detected.

FIGS. 33B to 33E each illustrate a structure example of a touchscreen.

The touchscreen 3532 illustrated in FIG. 33B includes a display panel3540 in which a display portion 3542 is provided between a firstsubstrate 3541 and a second substrate 3543, a third substrate 3545provided with a touch sensor 3544, and a protective substrate 3546.

As the display panel 3540, a variety of display devices such as adisplay device including a liquid crystal element or an organicelectroluminescence (EL) element and an electronic paper can be used.Note that the touchscreen 3532 may additionally include a backlight, apolarizing plate, and the like in accordance with the structure of thedisplay panel 3540.

An object comes in contact with or close to one of the surfaces of theprotective substrate 3546; thus, the mechanical strength of at least thesurface is preferably high. For example, a tempered glass which has beensubjected to physical or chemical treatment by an ion exchange method, athermal tempering method, or the like and has a surface to whichcompressive stress has been applied can be used as the protectivesubstrate 3546. Alternatively, a flexible substrate with a coatedsurface, such as a plastic substrate can be used. Note that a protectivefilm or an optical film may be provided over the protective substrate3546.

The touch sensor 3544 is provided on at least one of the surfaces of thethird substrate 3545. Alternatively, a pair of electrodes included inthe touch sensor 3544 may be formed on both surfaces of the thirdsubstrate 3545. A flexible film may be used as the third substrate 3545for thickness reduction of the touchscreen. The touch sensor 3544 may beheld between a pair of substrates (provided with a film).

Although the protective substrate 3546 and the third substrate providedwith the touch sensor 3544 are bonded to each other by a bonding layer3547 in FIG. 33B, the protective substrate 3546 and the third substrateare not necessarily bonded to each other. The third substrate 3545 andthe display panel 3540 may be bonded to each other by the bonding layer.

In the touchscreen 3532 illustrated in FIG. 33B, the display panel andthe substrate provided with the touch sensor are separately provided.The touchscreen having such a structure can also be referred to as anexternally attached touchscreen. In such a structure, the display paneland the substrate provided with the touch sensor are separately formedand then they are overlapped with each other, so that the display panelcan have a touch sensor function. Thus, the touchscreen can be easilymanufactured without a special manufacturing process.

In the touchscreen 3532 illustrated in FIG. 33C, the touch sensor 3544is provided on a surface of the second substrate 3543 which is on theprotective substrate 3546 side. The touchscreen having such a structurecan also be referred to as an on-cell touchscreen. With such astructure, the number of substrates needed can be reduced, which resultsin reductions in the thickness and weight of the touchscreen.

In the touchscreen 3532 illustrated in FIG. 33D, the touch sensor 3544is provided on one of the surfaces of the protective substrate 3546.With such a structure, the display panel and the touch sensor can beseparately manufactured; thus, the touchscreen can be easilymanufactured. Furthermore, the number of substrates needed can bereduced, which results in reductions in the thickness and weight of thetouchscreen.

In the touchscreen 3532 illustrated in FIG. 33E, the touch sensor 3544is provided between the pair of substrates in the display panel 3540.The touchscreen having such a structure can also be referred to as anin-cell touchscreen. With such a structure, the number of substratesneeded can be reduced, which results in reductions in the thickness andweight of the touchscreen. Such a touchscreen can be achieved, forexample, in such a manner that a circuit functioning as a touch sensoris formed using a transistor, a wiring, an electrode, and the likeincluded in the display portion 3542 on the first substrate 3541 or thesecond substrate 3543. Further, in the case of using an optical touchsensor, a photoelectric conversion element may be provided.

<Structural Example of in-Cell Touchscreen>

A structure example of a touchscreen incorporating the touch sensor intoa display portion including a plurality of pixels is described below.Here, an example where a liquid crystal element is used as a displayelement provided in the pixel is shown.

FIG. 34A is an equivalent circuit diagram of part of a pixel circuitprovided in the display portion of the touchscreen exemplified in thisstructure example.

Each pixel includes at least a transistor 3503 and a liquid crystalelement 3504. In addition, a gate of the transistor 3503 is electricallyconnected to a wiring 3501 and one of a source and a drain of thetransistor 3503 is electrically connected to a wiring 3502.

The pixel circuit includes a plurality of wirings extending in the Xdirection (e.g., a wiring 3510 _(—)1 and a wiring 3510_2) and aplurality of wirings extending in the Y direction (e.g., a wiring 3511).They are provided to intersect with each other, and capacitance isformed therebetween.

Among the pixels provided in the pixel circuit, ones of electrodes ofthe liquid crystal elements of some pixels adjacent to each other areelectrically connected to each other to form one block. The block isclassified into two types: an island-shaped block (e.g., a block 3515_1or a block 3515_2) and a linear block (e.g., a block 3516) extending inthe Y direction. Note that only part of the pixel circuit is illustratedin FIGS. 34A and 34B, and actually, these two kinds of blocks arerepeatedly arranged in the X direction and the Y direction.

The wiring 3510_1 (or 3510_2) extending in the X direction iselectrically connected to the island-shaped block 3515_1 (or the block3515_2). Although not illustrated, the wiring 3510_1 extending in the Xdirection is electrically connected to a plurality of island-shapedblocks 3515_1 which are provided discontinuously along the X directionwith the linear blocks therebetween. Further, the wiring 3511 extendingin the Y direction is electrically connected to the linear block 3516.

FIG. 34B is an equivalent circuit diagram in which a plurality ofwirings 3510 extending in the X direction and the plurality of wirings3511 extending in the Y direction are illustrated. Input voltage or acommon potential can be input to each of the wirings 3510 extending inthe X direction. Further, a ground potential can be input to each of thewirings 3511 extending in the Y direction, or the wirings 3511 can beelectrically connected to the detection circuit.

<Example of Operation of Touchscreen>

Operation of the above-described touchscreen is described with referenceto FIGS. 35A to 35C.

As illustrated in FIG. 35A, one frame period is divided into a writingperiod and a detecting period. The writing period is a period in whichimage data is written to a pixel, and the wirings 3510 (also referred toas gate lines) are sequentially selected. On the other hand, thedetecting period is a period in which sensing is performed by a touchsensor, and the wirings 3510 extending in the X direction aresequentially selected and input voltage is input.

FIG. 35B is an equivalent circuit diagram in the writing period. In thewiring period, a common potential is input to both the wiring 3510extending in the X direction and the wiring 3511 extending in the Ydirection.

FIG. 35C is an equivalent circuit diagram at some point in time in thedetection period. In the detection period, each of the wirings 3511extending in the Y direction is electrically connected to the detectioncircuit. Input voltage is input to the wirings 3510 extending in the Xdirection which are selected, and a common potential is input to thewirings 3511 extending in the X direction which are not selected.

It is preferable that a period in which an image is written and a periodin which sensing is performed by a touch sensor be separately providedas described above. Thus, a decrease in sensitivity of the touch sensorcaused by noise generated when data is written to a pixel can besuppressed.

Embodiment 11

In this embodiment, a driving method for reducing power consumption of adisplay device is described. By using the driving method in thisembodiment, power consumption of a display device including an oxidesemiconductor transistor in a pixel can be further reduced. Withreference to FIGS. 36 and 37, low power consumption of a liquid crystaldisplay device, which is an example of the display device, is describedbelow.

FIG. 36 is a block diagram illustrating a structural example of a liquidcrystal display device in this embodiment. As shown in FIG. 36, a liquidcrystal display device 500 includes a liquid crystal panel 501 as adisplay module, a control circuit 510, and a counter circuit.

An image signal (Video), which is digital data, and a synchronizationsignal (SYNC) for controlling rewriting of a screen of the liquidcrystal panel 501 are input to the liquid crystal display device 500.Examples of a synchronization signal include a horizontalsynchronization signal (Hsync), a vertical synchronization signal(Vsync), and a reference clock signal (CLK).

The liquid crystal panel 501 includes a display portion 530, a scan linedriver circuit 540, and a data line driver circuit 550. The displayportion 530 includes a plurality of pixels 531. The pixels 531 in thesame row are connected to the scan line driver circuit 540 through acommon scan line 541, and the pixels 531 in the same column areconnected to the data line driver circuit 550 through a common data line551.

A high power supply voltage (VDD) and a low power supply voltage (VSS),which serve as power supply voltages, and a common voltage (hereinafterreferred to as Vcom) are supplied to the liquid crystal panel 501. Thecommon voltage (Vcom) is supplied to each pixel 531 in the displayportion 530.

The data line driver circuit 550 processes an input image signal togenerate a data signal, and outputs the data signal to the data line551. The scan line driver circuit 540 outputs, to the scan line 541, ascan signal for selecting the pixel 531 into which a data signal is tobe written.

The pixel 531 includes a switching element whose electrical connectionto the data line 551 is controlled by a scan signal. When the switchingelement is turned on, a data signal is written into the pixel 531through the data line 551.

An electrode to which Vcom is applied corresponds to a common electrode.

The control circuit 510 controls the whole liquid crystal display device500 and includes a circuit which generates control signals for circuitsincluded in the liquid crystal display device 500.

The control circuit 510 includes a control circuit generation circuitwhich generates control signals for the scan line driver circuit 540 andthe data line driver circuit 550 on the basis of the synchronizationsignal (SYNC). Examples of a control signal for the scan line drivercircuit 540 include a start pulse (GSP) and a clock signal (GCLK).Examples of a control signal for the data line driver circuit 550include a start pulse (SSP) and a clock signal (SCLK). For example, thecontrol circuit 510 generates a plurality of clock signals with the samecycle and shifted phases as the clock signals (GCLK and SCLK).

Further, the control circuit 510 controls output of an image signal(Video), which is input from the outside of the liquid crystal displaydevice 500, to the data line driver circuit 550.

The data line driver circuit 550 includes a digital/analog conversioncircuit (hereinafter referred to as a D-A conversion circuit 552). TheD-A conversion circuit 552 converts an image signal to an analog signal,thereby generating a data signal.

Note that in the case where an image signal input to the liquid crystaldisplay device 500 is an analog signal, the image signal is converted toa digital signal in the control circuit 510 and output to the liquidcrystal panel 501.

An image signal is image data for each frame. The control circuit 510has a function of performing image processing on the image signal andcontrolling output of the image signal to the data line driver circuit550 on the basis of data obtained by the processing. For that function,the control circuit 510 includes a motion detection portion 511 whichdetects motion in the image data for each frame. The control circuit 510stops output of an image signal to the data line driver circuit 550 whenthe motion detection portion 511 determines that there is no motion, andrestarts the output of an image signal when the motion detection portion511 determines that there is motion.

There is no particular limitation on the image processing for detectingmotion which is performed in the motion detection portion 511. Anexample of a method for detecting motion is to obtain difference datafrom image data for two consecutive frames. It can be determined whetherthere is motion or not from the obtained difference data. Anotherexample of the method is to detect a motion vector.

In addition, the liquid crystal display device 500 may be provided withan image signal correction circuit which corrects an input image signal.For example, an image signal is corrected such that a voltage higherthan a voltage corresponding to the gray scale of the image signal iswritten into the pixel 531. Such correction can shorten the responsetime of the liquid crystal element. A method in which the controlcircuit 510 is driven with an image signal corrected in this manner isreferred to as overdriving. In the case of performing high frame ratedriving in which the liquid crystal display device 500 is driven at anintegral multiple of the frame frequency of an image signal, image datafor interpolation between two frames or image data for performing blackdisplay between two frames may be generated in the control circuit 510.

Next, the operation of the liquid crystal display device 500 fordisplaying an image with motion, such as a moving image, and an imagewithout motion, such as a still image, is described with reference to atiming chart in FIG. 37. FIG. 37 shows the signal waveforms of avertical synchronization signal (Vsync) and a data signal (Vdata) outputto the data line 551 from the data line driver circuit 550.

FIG. 37 is a timing chart of the liquid crystal display device 500during 3m frame periods. Here, there is motion in image data in thefirst k frame periods and the last j frame periods and there is nomotion in image data in the other frame periods. Note that k and j areeach an integer greater than or equal to 1 and less than or equal tom−2.

In the first k frame periods, the motion detection portion 511determines that there is motion in image data for each frame. Thecontrol circuit 510 outputs data signals (Vdata) to the data line 551 onthe basis of the result of determination by the motion detection portion511.

The motion detection portion 511 performs image processing for detectingmotion and determines that there is no motion in image data for the(k+1)-th frame. Then, the control circuit 510 stops output of imagesignals (Video) to the data line driver circuit 550 in the (k+1)-thframe period on the basis of the result of determination by the motiondetection portion 511. Thus, output of the data signal (Vdata) from thedata line driver circuit 550 to the data line 551 is stopped. Further,the control circuit 510 stops the supply of control signals (e.g., astart pulse signal and a clock signal) to the scan line driver circuit540 and the data line driver circuit 550 in order to stop rewriting ofthe display portion 530. The control circuit 510 does not output animage signal to the data line driver circuit 550 nor output controlsignals to the scan line driver circuit 540 and the data line drivercircuit 550, thereby keeping rewriting of the display portion 530stopped, until the motion detection portion 511 determines that there ismotion in image data.

Note that, in this specification, “not to supply” a signal to a liquidcrystal panel means to apply voltage which is different from apredetermined voltage for operating a circuit to a wiring for supplyingthe signal, or to bring the wiring into an electrically floating state.

When rewriting of the display portion 530 is stopped, an electric fieldin one direction is kept applied to the liquid crystal element, whichmight lead to deterioration of liquid crystal in the liquid crystalelement. In the case where such a problem is likely to occur, it ispreferable that signals be supplied to the scan line driver circuit 540and the data line driver circuit 550 from the control circuit 510 anddata signals with an inverted polarity be written into the data line 551at predetermined timings to invert the direction of the electric fieldapplied to the liquid crystal element, regardless of the result ofdetermination by the motion detection portion 511.

Note that the polarity of a data signal input to the data line 551 isdetermined relative to Vcom. The polarity is positive when the voltageof the data signal is higher than Vcom, and is negative when the voltageof the data signal is lower than Vcom.

Specifically, as shown in FIG. 37, in the (m+1)-th frame period, thecontrol circuit 510 outputs control signals to the scan line drivercircuit 540 and the data line driver circuit 550 and outputs an imagesignal (Video) to the data line driver circuit 550. The data line drivercircuit 550 outputs, to the data line 551, a data signal (Vdata) whichhas an inverted polarity with respect to a data signal (Vdata) output tothe data line 551 in the k-th frame period. In this manner, a datasignal (Vdata) with an inverted polarity is written into the data line551 in the (m+1)-th frame period and in the (2m+1)-th frame period,which are periods in which no motion is detected in image data.Rewriting of the display portion 530 is intermittently performed inperiods in which there is no change in image data; thus, it is possibleto reduce power consumption due to rewriting and prevent deteriorationof the liquid crystal element.

When the motion detection portion 511 determines that there is motion inimage data for any frame after the (2m+1)-th frame, the control circuit510 controls the scan line driver circuit 540 and the data line drivercircuit 550 to perform rewriting of the display portion 530.

As described above, with the driving method in FIG. 37, the polarity ofa data signal (Vdata) is inverted every m frame periods regardless ofwhether there is motion in image data (Video) or not. Meanwhile, thedisplay portion 530 is rewritten every frame in periods in which animage with motion is displayed and is rewritten every m frames inperiods in which an image without motion is displayed. Consequently,power consumed owing to rewriting of the display portion can be reduced.This can prevent an increase in power consumption due to an increase indriving frequency and the number of pixels.

As described above, in the liquid crystal device 500, the method fordriving the liquid crystal display device is switched between in amoving image display mode and in a still image display mode; thus, it ispossible to provide a liquid crystal display device with low powerconsumption while inhibiting deterioration of liquid crystal andmaintaining display quality.

In the case where a still image is displayed, when a pixel is rewrittenevery one frame, human eyes perceive the rewriting of the pixel asflickers in some cases, which causes eyestrain. The pixel is notfrequently rewritten in the display period of the still image in theliquid crystal device of this embodiment, which is effective forreducing eyestrain.

Thus, with the use of a liquid crystal panel in which a backplane isformed using an oxide semiconductor transistor, a middle size liquidcrystal display device with high resolution and low power consumption,which is very suitable for a portable electronic device, can beprovided.

Note that, in order to prevent deterioration of the liquid crystal, theinterval between polarity inversions of data signals (here, m frameperiods) is set to two seconds or shorter, preferably one second orshorter.

Although the detection of motion in image data is performed in themotion detection portion 511 in the control circuit 510, the detectionof motion is not necessarily performed only in the motion detectionportion 511. Data on whether there is motion or not may be input to thecontrol circuit 510 from the outside of the liquid crystal displaydevice 500.

Determination that there is no motion in image data is not always basedon image data for two consecutive frames; the number of frames requiredfor the determination may be set as appropriate depending on the usagemode of the liquid crystal display device 500. For example, rewriting ofthe display portion 530 may be stopped when there is no motion in imagedata for m consecutive frames.

Note that although description of this embodiment is made using a liquidcrystal display device as a display device, the driving method in thisembodiment can be used for other display devices, e.g., a light-emittingdisplay device.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in the other embodiments andexamples.

Embodiment 12

The semiconductor device which is one embodiment of the presentinvention can be applied to a variety of electronic appliances(including game machines). Examples of electronic appliances include atelevision device (also referred to as television or televisionreceiver), a monitor of a computer or the like, a digital camera, adigital video camera, a digital photo frame, a mobile phone, a portablegame machine, a portable information terminal, an audio reproducingdevice, a game machine (e.g., a pachinko machine or a slot machine), anda game console, and the like. Examples of these electronic appliancesare illustrated in FIGS. 38A to 38C.

FIG. 38A illustrates a table 9000 having a display portion. In the table9000, a display portion 9003 is incorporated in a housing 9001 and animage can be displayed on the display portion 9003. The housing 9001 issupported by four leg portions 9002. Further, a power cord 9005 forsupplying power is provided for the housing 9001.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9003. Thus, the display portion 9003 canhave high display quality.

The display portion 9003 has a touch-input function. When a user touchesdisplayed buttons 9004 which are displayed on the display portion 9003of the table 9000 with his/her fingers or the like, the user can carryout operation of the screen and input of information. Further, when thetable may be made to communicate with home appliances or control thehome appliances, the display portion 9003 may function as a controldevice which controls the home appliances by operation on the screen.For example, with the use of the semiconductor device having an imagesensor function, the display portion 9003 can have a touch-inputfunction.

Further, the screen of the display portion 9003 can be placedperpendicular to a floor with a hinge provided for the housing 9001;thus, the table can also be used as a television device. When atelevision device having a large screen is set in a small room, an openspace is reduced; however, when a display portion is incorporated in atable, a space in the room can be efficiently used.

FIG. 38B illustrates a television device 9100. In the television device9100, a display portion 9103 is incorporated in a housing 9101 and animage can be displayed on the display portion 9103. Note that thehousing 9101 is supported by a stand 9105 here.

The television device 9100 can be operated with an operation switch ofthe housing 9101 or a separate remote controller 9110. Channels andvolume can be controlled with an operation key 9109 of the remotecontroller 9110 so that an image displayed on the display portion 9103can be controlled. Furthermore, the remote controller 9110 may beprovided with a display portion 9107 for displaying data output from theremote controller 9110.

The television device 9100 illustrated in FIG. 38B is provided with areceiver, a modem, and the like. With the receiver, general televisionbroadcasts can be received in the television device 9100. Further, whenthe television device 9100 is connected to a communication network bywired or wireless connection via the modem, one-way (from a transmitterto a receiver) or two-way (between a transmitter and a receiver orbetween receivers) data communication can be performed.

Any of the semiconductor devices described in the above embodiments canbe used for the display portions 9103 and 9107. Thus, the televisiondevice can have high display quality.

FIG. 38C illustrates a computer 9200, which includes a main body 9201, ahousing 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, a pointing device 9206, and the like.

Any of the semiconductor devices described in the above embodiments canbe used for the display portion 9203. Thus, the computer 9200 can havehigh display quality.

The display portion 9203 has a touch-input function. When a user touchesdisplayed buttons which are displayed on the display portion 9203 of thecomputer 9200 with his/her fingers or the like, the user can carry outoperation of the screen and input of information. Further, when thecomputer may be made to communicate with home appliances or control thehome appliances, the display portion 9203 may function as a controldevice which controls the home appliances by operation on the screen.

FIGS. 39A and 39B illustrate a foldable tablet terminal. In FIG. 39A,the tablet terminal is opened and includes a housing 9630, a displayportion 9631 a, a display portion 9631 b, a display-mode switchingbutton 9034, a power button 9035, a power-saving-mode switching button9036, a clip 9033, and an operation button 9038.

Any of the semiconductor devices described in the above embodiments canbe used for the display portion 9631 a and the display portion 963 lb.Thus, the display quality of the tablet terminal can be improved.

Part of the display portion 9631 a can be a touch panel region 9632 aand data can be input when a displayed operation key 9638 is touched.Although a structure in which a half region in the display portion 9631a has only a display function and the other half region also has a touchpanel function is illustrated as an example, the structure of thedisplay portion 9631 a is not limited thereto. The whole area of thedisplay portion 9631 a may have a touch screen function. For example,the whole area of the display portion 9631 a can display keyboardbuttons and serve as a touch screen while the display portion 9631 b canbe used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch screen region 9632 b. When a keyboard display switchingbutton 9639 displayed on the touch panel is touched with a finger, astylus, or the like, a keyboard can be displayed on the display portion963 lb.

Touch input can be performed concurrently on the touch screen regions9632 a and 9632 b.

The display-mode switching button 9034 can switch display orientation(e.g., between landscape mode and portrait mode) and select a displaymode (switch between monochrome display and color display), for example.The power-saving-mode switching button 9036 can control displayluminance in accordance with the amount of external light in use of thetablet terminal detected by an optical sensor incorporated in thetablet. The tablet terminal may include another detection device such asa sensor for detecting orientation (e.g., a gyroscope or an accelerationsensor) in addition to the optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 39A, one embodiment of the presentinvention is not limited to this example. The display portion 9631 a andthe display portion 9631 b may have different areas or different displayquality. For example, one of them may be a display panel that candisplay higher-definition images than the other.

In FIG. 39B, the tablet terminal is folded and includes the housing9630, a solar cell 9633, and a charge and discharge control circuit9634. Note that in FIG. 39B, an example in which the charge anddischarge control circuit 9634 includes the battery 9635 and the DCDCconverter 9636 is illustrated.

Since the tablet can be folded in two, the housing 9630 can be closedwhen not in use. Thus, the display portions 9631 a and 9631 b can beprotected, thereby providing a tablet with high endurance and highreliability for long-term use.

In addition, the tablet terminal illustrated in FIGS. 39A and 39B canhave a function of displaying various kinds of data (e.g., a stillimage, a moving image, and a text image), a function of displaying acalendar, a date, the time, or the like on the display portion, atouch-input function of operating or editing the data displayed on thedisplay portion by touch input, a function of controlling processing bya variety of kinds of software (programs), and the like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch screen, a display portion,an image signal processor, and the like. Note that the solar cell 9633can be provided on one or both surfaces of the housing 9630, so that thebattery 9635 can be charged efficiently. The use of a lithium ionbattery as the battery 9635 is advantageous in downsizing or the like.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 39B are described with reference to a blockdiagram of FIG. 39C. The solar cell 9633, the battery 9635, the DCDCconverter 9636, a converter 9637, switches SW1 to SW3, and the displayportion 9631 are shown in FIG. 39C, and the battery 9635, the DCDCconverter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge/discharge control circuit 9634 in FIG. 39B.

First, an example of the operation in the case where power is generatedby the solar cell 9633 using external light is described. The voltage ofpower generated by the solar battery is raised or lowered by the DCDCconverter 9636 so that the power has a voltage for charging the battery9635. Then, when the power from the solar cell 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 9637 soas to be a voltage needed for the display portion 9631. In addition,when display on the display portion 9631 is not performed, the switchSW1 is turned off and a switch SW2 is turned on so that charge of thebattery 9635 may be performed.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, without limitation thereon, the battery 9635may be charged using another power generation means such as apiezoelectric element or a thermoelectric conversion element (Peltierelement). For example, the battery 9635 may be charged with anon-contact power transmission module that transmits and receives powerwirelessly (without contact) to charge the battery or with a combinationof other charging means.

Note that the structures and the like described in this embodiment canbe combined as appropriate with any of the structures and the likedescribed in the other embodiments.

Example 1

Example 1 describes measurement results of Vg-Id characteristics oftransistors and BT photostress tests.

First of all, a manufacturing process of a transistor included in asample 1 is described. In this example, the process is described withreference to FIGS. 2A to 2D.

First, as illustrated in FIG. 2A, a glass substrate was used as thesubstrate 11, and the gate electrode 15 was formed over the substrate11.

A 100-nm-thick tungsten film was formed by a sputtering method, a maskwas formed over the tungsten film by a photolithography process, andpart of the tungsten film was etched with the use of the mask, so thatthe gate electrode 15 was formed.

Next, the gate insulating film 17 (corresponding to GI in FIG. 40) wasformed over the gate electrode 15.

The gate insulating film 17 was formed by stacking a 50-nm-thick firstsilicon nitride film and a 200-nm-thick silicon oxynitride film.

The silicon nitride film was formed under the following conditions:silane with a flow rate of 50 sccm and nitrogen with a flow rate of 5000sccm were supplied to a treatment chamber of a plasma CVD apparatus asthe source gas; the pressure in the treatment chamber was controlled to60 Pa; and a power of 150 W was supplied with the use of a 27.12 MHzhigh-frequency power source.

Next, the silicon oxynitride film was formed under the followingconditions: silane with a flow rate of 20 sccm and dinitrogen monoxidewith a flow rate of 3000 sccm were supplied to the treatment chamber ofthe plasma CVD apparatus as the source gas; the pressure in thetreatment chamber was controlled to 40 Pa, and a power of 100 W wassupplied with the use of a 27.12 MHz high-frequency power source.

In each of the forming steps of the silicon nitride film and the siliconoxynitride film, the substrate temperature was 350° C.

Next, the oxide semiconductor film 18 was formed to overlap with thegate electrode 15 with the gate insulating film 17 providedtherebetween.

Here, a 35-nm-thick oxide semiconductor film was formed over the gateinsulating film 17 by a sputtering method. Then, a mask was formed overthe oxide semiconductor film by a photolithography process, and part ofthe oxide semiconductor film was etched using the mask to form the oxidesemiconductor film 18 (corresponding to S1 in FIG. 40).

The oxide semiconductor film (S1) was formed under the followingconditions: a sputtering target where In:Ga:Zn=1:1:1 (atomic ratio) wasused; argon with a flow rate of 100 sccm and oxygen with a flow rate of100 sccm were supplied as a sputtering gas into a treatment chamber of asputtering apparatus; the pressure in the treatment chamber wascontrolled to 0.6 Pa; and a direct-current power of 5 kW was supplied.Note that the oxide semiconductor film was formed at a substratetemperature of 170° C.

For the structure obtained through the steps up to here, FIG. 2B can bereferred to.

Next, after the gate electrode was exposed by partly etching the gateinsulating film 17 (not illustrated), the pair of electrodes 21 and 22in contact with the oxide semiconductor film 18 was formed asillustrated in FIG. 2C.

Here, a conductive film was formed over the gate insulating film 17 andthe oxide semiconductor film 18. As the conductive film, a 400-nm-thickaluminum film was formed over a 50-nm-thick tungsten film, and a100-nm-thick titanium film was formed over the aluminum film. Then, amask was formed over the conductive film by a photolithography process,and part of the conductive film was subjected to wet etching using themask, whereby the pair of electrodes 21 and 22 was formed.

Next, after the substrate was transferred to a treatment chamber in areduced pressure and heated at 350° C., the oxide semiconductor film 18was exposed to oxygen plasma that was generated in a dinitrogen monoxideatmosphere by supply of a high-frequency power of 150 W to an upperelectrode provided in the treatment chamber with the use of a 27.12 MHzhigh-frequency power source.

Then, the protective film 26 was formed over the oxide semiconductorfilm 18 and the pair of electrodes 21 and 22 (see FIG. 2D). Here, as theprotective film 26, the oxide insulating film 23 (corresponding to P1 inFIG. 40) and the oxide insulating film 24 (corresponding to P2 in FIG.40) were formed.

First, after the above plasma treatment, the oxide insulating film 23and the oxide insulating film 24 were formed in succession withoutexposure to the atmosphere. A 10-nm-thick silicon oxynitride film wasformed as the oxide insulating film 23, and a 390-nm-thick siliconoxynitride film was formed as the oxide insulating film 24.

The oxide insulating film 23 was formed by a plasma CVD method under thefollowing conditions: silane with a flow rate of 20 sccm and dinitrogenmonoxide with a flow rate of 3000 sccm were used as a source gas; thepressure in the treatment chamber was 200 Pa; the substrate temperaturewas 350° C.; and a high-frequency power of 100 W was supplied toparallel-plate electrodes.

The oxide insulating film 24 was formed by a plasma CVD method under thefollowing conditions: silane with a flow rate of 160 sccm and dinitrogenmonoxide with a flow rate of 4000 sccm were used as a source gas, thepressure in the treatment chamber was 200 Pa, the substrate temperaturewas 220° C., and the high-frequency power of 1500 W was supplied to theparallel-plate electrodes. Under the above conditions, it is possible toform a silicon oxynitride film which contains oxygen at a higherproportion than the stoichiometric composition and from which part ofoxygen is released by heating.

Next, by heat treatment, water, nitrogen, hydrogen, or the like wasreleased from the oxide insulating film 23 and the oxide insulating film24 and part of oxygen contained in the oxide insulating film 24 wassupplied to the oxide semiconductor film 18. Here, the heat treatmentwas performed in an atmosphere of nitrogen and oxygen at 350° C. for onehour.

Next, although not illustrated, an opening which exposes part of thepair of electrodes 21 and 22 was formed by partly etching the protectivefilm 26.

Next, a planarization film was formed (not illustrated) over theprotective film 26. Here, the protective film 26 was coated with acomposition, and exposure and development were performed, so that aplanarization film having an opening through which the pair ofelectrodes is partly exposed was formed. Note that as the planarizationfilm, a 1.5-μm-thick acrylic resin was formed. Then, heat treatment wasperformed. The heat treatment was performed at a temperature of 250° C.in a nitrogen atmosphere for one hour.

Next, a conductive film connected to part of the pair of electrodes wasformed (not illustrated). Here, a 100-nm-thick ITO film containingsilicon oxide was formed as the conductive film by a sputtering method.After that, heat treatment was performed at 250° C. in a nitrogenatmosphere for one hour.

Through the above process, the sample 1 including a transistor wasformed.

Another sample was formed in the following manner. The gate insulatingfilm 17 in the transistor of the sample 1 was formed by stacking a50-nm-thick first silicon nitride film, a 300-nm-thick second siliconnitride film, a 50-nm-thick third silicon nitride film, and a50-nm-thick silicon oxynitride film. Further, instead of the oxidesemiconductor film 18, a multilayer film in which a 35-nm-thick oxidesemiconductor film (corresponding to S1 in FIG. 40) and a 10-nm-thickoxide film (corresponding to S2 in FIG. 40) were stacked was formed.After the pair of electrodes was formed, a surface of the multilayerfilm was subjected to cleaning treatment using a phosphoric acidsolution in which 85% phosphoric acid was diluted by 100 times. Theprotective film 26 was formed by stacking a 10-nm-thick oxide insulatingfilm 23, a 400-nm-thick oxide insulating film 24, and a 100-nm-thicksilicon nitride film 25 (corresponding to P3 in FIG. 40). A samplehaving such a structure is referred to as a sample 2.

The film formation conditions of the first to the third silicon nitridefilms of the gate insulating film 17 in the sample 2 are describedbelow.

The first silicon nitride film was formed under the followingconditions: silane with a flow rate of 200 sccm, nitrogen with a flowrate of 2000 sccm, and ammonia with a flow rate of 100 sccm weresupplied to the treatment chamber of the plasma CVD apparatus as thesource gas; the pressure in the treatment chamber was controlled to 100Pa, and a power of 2000 W was supplied with the use of a 27.12 MHzhigh-frequency power source.

Next, the second silicon nitride film was formed in such a manner that,in the conditions of the source gas of the first silicon nitride film,the flow rate of ammonia was changed to 2000 sccm.

Next, the third silicon nitride film was formed under the followingconditions: silane with a flow rate of 200 sccm and nitrogen with a flowrate of 5000 sccm were supplied to the treatment chamber of the plasmaCVD apparatus as the source gas; the pressure in the treatment chamberwas controlled to 100 Pa, and the power of 2000 W was supplied with theuse of a 27.12 MHz high-frequency power source.

The film formation conditions of the oxide film (S2) in contact with theoxide semiconductor film 18 in the sample 2 are described below. Theoxide film (S2) was formed under the following conditions: a sputteringtarget where In:Ga:Zn=1:3:2 (atomic ratio) was used; Ar with a flow rateof 180 sccm and oxygen with a flow rate of 20 sccm were supplied as asputtering gas into the treatment chamber of the sputtering apparatus;the pressure in the treatment chamber was controlled to 0.6 Pa; and adirect-current power of 5 kW was supplied. Note that the oxide film wasformed at a substrate temperature of 170° C.

The film formation conditions of the nitride insulating film 25(corresponding to P3 in FIG. 40) in the sample 2 are described below.The nitride insulating film 25 was formed by a plasma CVD method underthe following conditions: silane with a flow rate of 50 sccm, nitrogenwith a flow rate of 5000 sccm, and ammonia with a flow rate of 100 sccmwere used as a source gas, the pressure in the treatment chamber was 100Pa, the substrate temperature was 350° C., and a high-frequency power of1000 W was supplied to the parallel-plate electrodes.

Another sample was formed in the following manner. In the transistor ofthe sample 1, the gate insulating film 17 was formed using the structureand conditions similar to those of the sample 2. Further, after theoxide semiconductor film 18 was formed, heat treatment was performed at450° C. Note that in a manner similar to that of the sample 2, after thepair of electrodes was formed, a surface of the oxide semiconductor film18 was subjected to cleaning treatment using a phosphoric acid solutionin which 85% phosphoric acid was diluted by 100 times. The thickness ofthe oxide insulating film 23 was 50 nm. In the film formation conditionsof the oxide insulating film 23 (corresponding to P1 in FIG. 40), thefilm formation temperature was 220° C. Further, the protective film wasformed by stacking the oxide insulating film 23, the oxide insulatingfilm 24, and the nitride insulating film 25 (corresponding to P3 in FIG.40) in a manner similar to that of the sample 2. A sample having such astructure is referred to as a comparative sample 1.

A comparative sample 2 was formed in such a manner that, in thecomparative sample 1, the heat treatment was performed at 350° C. afterthe formation of the oxide semiconductor film 18.

A comparative sample 3 was formed in the following manner. In thecomparative sample 1, instead of the oxide semiconductor film 18, themultilayer film including the oxide semiconductor film 18 and the oxidefilm was formed using the structure and conditions similar to those ofthe sample 2.

A transistor included in each sample has a channel length (L) of 6 μmand a channel width (W) of 50 μm.

<Vg-Id Characteristics>

Next, initial Vg-Id characteristics of the transistors included in thesamples 1 and 2 and the comparative samples 1 and 2 were measured. Here,change in characteristics of current flowing between a source electrodeand a drain electrode (hereinafter referred to as the drain current),that is, Vg-Id characteristics were measured under the followingconditions: the substrate temperature was 25° C., the potentialdifference between the source electrode and the drain electrode(hereinafter referred to as the drain voltage) was 1 V or 10 V, and thepotential difference between the source electrode and the gate electrode(hereinafter referred to as the gate voltage) were changed from −15 V to+20 V.

FIG. 40 shows the Vg-Id characteristics of the transistors included inthe samples. In each graph shown in FIG. 40, the horizontal axisindicates gate voltage Vg, the left vertical axis indicates draincurrent Id, and the right vertical axis indicates field-effect mobility.Note that a voltage range of −15 V to 15 V is represented along thehorizontal axis. Further, the solid lines indicate the Vg-Idcharacteristics at the drain voltages Vd of 1 V and 10 V, and the dashedlines indicate the field-effect mobility with respect to the gatevoltages at the drain voltage Vd of 10 V. Note that the field effectmobility was obtained by operation of each sample in a saturationregion.

Further, in each of the samples, 20 transistors having the samestructure were formed on the substrate.

The results in FIG. 40 show that favorable switching characteristics canbe obtained in each of the samples 1 and 2 and the comparative samples 1and 2.

Next, a BT stress test and a BT photostress test were performed on eachof the samples 1 and 2 and the comparative samples 1 and 2. The BTstress test is one kind of accelerated test and can evaluate, in a shorttime, a change in characteristics (i.e., a change with time) of atransistor, which is caused by long-term use. The amount of change incharacteristics of the transistor before and after the BT stress test isan important indicator when examining the reliability of the transistor.

<Gate BT Stress Test and Gate BT Photostress Test>

A gate BT stress test and a gate BT photostress test were performed.

A measurement method of the gate BT stress test is described. First,initial Vg-Id characteristics of the transistor were measured asdescribed above.

Next, the substrate temperature was set at a given temperature(hereinafter referred to as stress temperature) and the temperature waskept constant, the pair of electrodes serving as a source electrode anda drain electrode of the transistor were set at a same potential, andthe gate electrode was supplied for a certain period of time(hereinafter referred to as stress time) with potential different fromthat of the pair of electrodes serving as a source electrode and a drainelectrode. Next, the substrate temperature was set as appropriate, andthe electrical characteristics of the transistor were measured. As aresult, a difference in threshold voltage and a difference in shiftvalue between before and after the gate BT stress test can be obtainedas the amount of change in the electrical characteristics.

Note that a stress test where negative voltage is applied to a gateelectrode is called negative gate BT stress test (Dark×GBT); whereas astress test where positive voltage is applied is called positive gate BTstress test (Dark+GBT). Note that a stress test where negative voltageis applied to a gate electrode while light emission is performed iscalled negative gate BT photostress test (Photo−GBT); whereas a stresstest where positive voltage is applied while light emission is performedis called positive gate BT photostress test (Photo+GBT).

Here, the gate BT stress conditions were as follows: stress temperature,60° C.; stress time, 3600 seconds; voltage applied to the gateelectrode, −30 V or +30 V; voltage applied to the source electrode, 0 V;and voltage applied to the drain electrode, 0 V. The electric fieldintensity applied to the gate insulating film was 0.66 MV/cm.

Under conditions similar to those of the above BT stress test, the gateBT photostress test where the transistor is irradiated with white LEDlight with 10000 lx was performed. Note that the Vg-Id characteristicsof the transistor after the BT stress test were measured at atemperature of 60° C.

FIG. 41A shows a difference between threshold voltage in the initialcharacteristics and threshold voltage after the BT stress test (i.e.,the amount of change in threshold voltage (ΔVth)) and a difference inshift value (i.e. the amount of change in shift value (ΔShift)) of eachof the transistors included in the samples 1 and 2 and the comparativesamples 1 and 2. FIG. 41A shows the amounts of change due to thepositive gate BT stress test (Dark+GBT), the negative gate BT stresstest (Dark−GBT), the positive gate BT photostress test (Photo+GBT), andthe negative gate BT photostress test (Photo−GBT).

Next, the stress temperature of a stress test was changed. Performedhere is a gate BT stress test where the stress temperature was changedto 125° C. in the conditions of the above-described gate BT stress test.Note that the Vg-Id characteristics of the transistor after the gate BTstress test were measured at a temperature of 40° C.

FIG. 41B shows the amounts of change in threshold voltage (ΔVth) and theamounts of change in shift value (ΔShift) of the samples 1 and 2 and thecomparative samples 1 and 2. In FIG. 41B, the amounts of change by apositive gate BT stress test (Dark+GBT) and a negative gate BT stresstest (Dark−GBT) are shown.

Here, a threshold voltage and a shift value in this specification aredescribed with reference to FIGS. 42A and 42B.

In this specification, in a curve 612 where the horizontal axis and thevertical axis indicate the gate voltage (Vg [V]) and the square root ofdrain current (Id^(1/2) [A]), respectively, the threshold voltage (Vth)is defined as a gate voltage at a point of intersection of anextrapolated tangent line 614 of Id^(1/2) having the highest inclinationwith the Vg axis (i.e., Id^(1/2) of 0 A) (see FIG. 42A). Note that inthis specification, threshold voltage is calculated with a drain voltageVd of 10 V. Further, in this specification, threshold voltage (Vth)refers to an average value of Vth of 20 transistors included in eachsample.

In this specification, in a curve 616 where the horizontal axis and thevertical axis indicate the gate voltage (Vg [V]) and the logarithm ofdrain current (Id [A]), respectively, the shift value (Shift) is definedas a gate voltage at a point of intersection of an extrapolated tangentline 618 of Id having the highest inclination with a straight line ofId=1.0×10⁻¹² [A] (see FIG. 42B). Note that in this specification, ashift value is calculated with a drain voltage Vd of 10 V. Further, inthis specification, the shift value refers to an average value of shiftvalues of 20 transistors included in each sample.

It is shown from FIG. 41A that the amount of change in each of thesamples 1 and 2 due to the positive gate BT stress test (Dark+GBT) andthe negative gate BT stress test (Dark−GBT) is smaller than that of eachof the comparative samples 1 and 2 in the case where the stresstemperature is 60° C.

It is shown from FIG. 41B that the amount of change in each of thesamples 1 and 2 due to the positive gate BT stress test (Dark+GBT) andthe negative gate BT stress test (Dark−GBT) is smaller than that of eachof the comparative samples 1 and 2 in the case where the stresstemperature is 120° C.

The above results show that, when the oxide insulating film is formedover the oxide semiconductor film or the multilayer film at atemperature of higher than or equal to 280° C. and lower than or equalto 400° C., the impurity can be released from the oxide semiconductorfilm or the multilayer film without performing heat treatment after theoxide semiconductor film or the multilayer film is formed. Thus, theamount of change in transistor characteristics can be reduced.

Further, a positive gate BT stress test (Dark+GBT) was performed on thesamples 1 and 2 and the comparative samples 1 and 3. Here, the stresstemperature was set to 60° C. or 125° C. and the stress time was set to100 seconds, 500 seconds, 1500 seconds, 2000 seconds, and 3600 secondsto measure the amount of change in threshold voltage. FIGS. 43A and 43Bshow the amounts of change in threshold voltage in each stress time andan approximate line obtained from the amounts of change. The horizontalaxis indicates stress time and the vertical axis indicates the amount ofchange in threshold voltage (ΔVth). FIG. 43A shows the measurementresults when the stress temperature is 60° C. FIG. 43B shows themeasurement results when the stress temperature is 125° C.

It is shown from FIG. 43A that the amount of change in threshold voltageof each of the samples 1 and 2 is smaller than that of the comparativesample 1. These results show that, when the oxide insulating film isformed over the oxide semiconductor film or the multilayer film at atemperature of higher than or equal to 280° C. and lower than or equalto 400° C., the amount of change in transistor characteristics can bereduced without performing heat treatment after the oxide semiconductorfilm or the multilayer film is formed.

Further, it is shown from FIGS. 43A and 43B that the amounts of changein transistor characteristics of the samples 1 and 2 are larger than,but substantially equal to, that of the comparative sample 3.

Example 2

Example 2 describes the amounts of water and oxygen released from theoxide insulating film 23 and the oxide insulating film 24 in Embodiment1 and the amount of defects in the films.

First, samples each including an oxide insulating film were measured byTDS to evaluate the amounts of released water and oxygen.

First of all, a process for manufacturing the samples is described.

A silicon oxynitride film was formed on a silicon wafer by a plasma CVDmethod using the conditions for forming the oxide insulating film 23described in Embodiment 1. The sample is referred to as a sample 3. Notethat the thickness of the silicon oxynitride film included in the sample3 was 100 nm.

The silicon oxynitride film included in the sample 3 was formed underthe following conditions: silane with a flow rate of 20 sccm anddinitrogen monoxide with a flow rate of 3000 sccm were supplied to thetreatment chamber of the plasma CVD apparatus as the source gas; thepressure in the treatment chamber was controlled to 200 Pa; thesubstrate temperature was 350° C.; and a power of 100 W was suppliedwith the use of a 27.12 MHz high-frequency power source.

Another sample was formed in such a manner that a silicon oxynitridefilm was formed on a silicon wafer by a plasma CVD method using theconditions for forming the oxide insulating film 24 described inEmbodiment 1. The sample is referred to as a sample 4. Note that thethickness of the silicon oxynitride film included in the sample 4 was400 nm.

The silicon oxynitride film included in the sample 4 was formed by aplasma CVD method under the following conditions: silane with a flowrate of 160 sccm and dinitrogen monoxide with a flow rate of 4000 sccmwere used as a source gas; the pressure in the treatment chamber was 200Pa; the substrate temperature was 220° C.; and a high-frequency power of1500 W was supplied to the parallel-plate electrodes. Under the aboveconditions, it is possible to form a silicon oxynitride film whichcontains oxygen at a higher proportion than the stoichiometriccomposition and from which part of oxygen is released by heating.

Another sample was formed in such a manner that a silicon oxynitridefilm was formed on a silicon wafer by a plasma CVD method usingconditions where the film formation pressure and the film formationtemperature are lower than those for the sample 3. The sample isreferred to as a comparative sample 4. Note that the thickness of thesilicon oxynitride film included in the comparative sample 4 was 400 nm.

The silicon oxynitride film included in the comparative sample 4 wasformed under the following conditions: silane with a flow rate of 30sccm and dinitrogen monoxide with a flow rate of 4000 sccm were suppliedto the treatment chamber of the plasma CVD apparatus as the source gas;the pressure in the treatment chamber was controlled to 40 Pa; thesubstrate temperature was 220° C.; and a power of 150 W was suppliedwith the use of a 27.12 MHz high-frequency power source.

<TDS Measurement>

FIG. 44 shows the results of TDS measurement performed on the samples 3and 4 and the comparative sample 4. Shown in the upper part of FIG. 44are measurement results indicating the amount of released watermolecules. Shown in the lower part of FIG. 44 are measurement resultsindicating the amount of released oxygen molecules.

As shown in the upper part of FIG. 44, a peak of M/z=18 corresponding tothe mass number of a water molecule is observed in the comparativesample 4. Further, peak intensities in the vicinity of substratetemperatures from 50° C. to 150° C. of the samples 3 and 4 are lowerthan those of the comparative sample 4. Thus, the amount of water issmall in each of the films formed using the conditions for forming theoxide insulating film 23 and the oxide insulating film 24 in Embodiment1.

As shown in the lower part of FIG. 44, a peak of M/z=32 corresponding tothe mass number of an oxygen molecule is observed in the sample 4.Further, peak intensities in the vicinity of substrate temperatures from300° C. to 400° C. of the sample 3 and the comparative sample 4 arelower than those of the sample 4. Thus, the amount of water is small inthe film formed using the conditions for forming the oxide insulatingfilm 24 in Embodiment 1.

Next, description is made on the results of measuring, by electron spinresonance (ESR), the amounts of defects in the oxide insulating filmsincluded in the samples 3 and 4 and the comparative sample 4.

First, the structures of evaluated samples are described.

A sample 5 was formed in such a manner that the silicon oxynitride filmincluded in the sample 3 was formed over a quartz substrate. Note thatthe thickness of the silicon oxynitride film included in the sample 5was 100 nm.

A sample 6 was formed in such a manner that the silicon oxynitride filmincluded in the sample 4 was formed over a quartz substrate. Note thatthe thickness of the silicon oxynitride film included in the sample 6was 400 nm.

A comparative sample 5 was formed in such a manner that the siliconoxynitride film included in the comparative sample 4 was formed over aquartz substrate. Note that the thickness of the silicon oxynitride filmincluded in the comparative sample 5 was 400 nm

<ESR Measurement>

Next, ESR measurement was performed on the samples 5 and 6 and thecomparative sample 5. In the ESR measurement performed at apredetermined temperature, a value of a magnetic field (H₀) where amicrowave is absorbed is used for an equation g=hv/βH₀, so that aparameter of a g-factor can be obtained. Note that v represents thefrequency of the microwave. Note that h and β represent the Planckconstant and the Bohr magneton, respectively, and are both constants.

Here, the ESR measurement was performed under the following conditions.The measurement temperature was −170° C., the high-frequency power(power of microwaves) of 8.92 GHz was 1 mW, and the direction of amagnetic field was parallel to a surface of each sample. Note that thelower limit of the detection of the spin density of a signal whichappears at g (g-factor)=2 due to a dangling bond of silicon was 1.1×10¹¹spins. The smaller the number of spins is, the fewer the defects thatare dangling bonds of silicon.

First derivative curves obtained by performing ESR measurement on thesamples are shown in the upper part of FIG. 45. The spin densities ofsignals which appear at g (g-factor)=2 due to dangling bonds of siliconin the samples are shown in the lower part of FIG. 45. Note that shownhere is spin density obtained by converting the number of measured spinsinto that per unit volume.

Note that here, ESR measurement was performed on the sample before andafter heat treatment to determine change in the amount of defects due toheat treatment. In FIG. 45, “as-depo” is written beside the result ofmeasurement before heat treatment, and “350° C.” is written beside theresult of measurement after heat treatment at 350° C.

As shown in the upper part of FIG. 45, in the silicon oxynitride filmincluded in the sample 5, a signal having symmetry is not detected at ag-factor of 2 before and after the heat treatment. Thus, it is shownthat, in the silicon oxynitride film included in the sample 5, theamount of defects is extremely low or no defect is included.

In the silicon oxynitride film included in each of the sample 6 and thecomparative sample 5, a signal having symmetry is detected at a g-factorof 2 before the heat treatment. Thus, it is revealed that a defect iscontained in the silicon oxynitride film included in each of the sample6 and the comparative sample 5. In the sample 6, a signal havingsymmetry is detected at a g-factor of 2 after the heat treatment, but inthe comparative sample 5, a signal having symmetry is not detected at ag-factor of 2 after the heat treatment. Thus, in the comparative sample5, the amount of defects in the film is reduced or no defect is includedowing to the heat treatment.

The above results show that the oxide insulating film with few defectscan be formed using the conditions for forming the oxide insulating film23 described in Embodiment 1.

Example 3

Example 3 describes the relation between the film formation temperatureof the oxide insulating film 23 described in Example 1 and theconcentration of hydrogen contained in the oxide semiconductor film andthe oxide insulating film. In this example, hydrogen concentration wasmeasured by SIMS measurement performed on a sample in which the oxidesemiconductor film and the oxide insulating film were stacked.

First of all, a process for manufacturing the sample is described.

A 100-nm-thick oxide semiconductor film (corresponding to OS in FIG. 46)was formed over a quartz substrate by a sputtering method. Next, heattreatment was performed.

Here, an oxide semiconductor film was formed using conditions similar tothose for the oxide semiconductor film included in the sample 1 inExample 1. Further, after performing heat treatment at 350° C. in anitrogen atmosphere for one hour, heat treatment was performed at 350°C. in an atmosphere containing nitrogen and oxygen for one hour.

Next, a 20-nm-thick silicon oxynitride film (corresponding to P1 in FIG.46) was formed over the oxide semiconductor film using the conditionsfor forming the oxide insulating film 23 described in Embodiment 1, andafter that, a 200-nm-thick silicon oxynitride film (corresponding to P2in FIG. 46) was formed under the conditions for forming the oxideinsulating film 24 described in Embodiment 1.

Here, silicon oxynitride (P1) was formed under conditions similar tothose for the oxide insulating film 23 included in the sample 1described in Example 1. Silicon oxynitride (P2) was formed underconditions similar to those for the oxide insulating film 23 included inthe sample 1 described in Example 1. The sample is referred to as asample 7.

Further, a comparative sample 6 was formed in the following manner: inthe heat treatment performed after the formation of the oxidesemiconductor film (corresponding to OS in FIG. 46) of the sample 7, theheat treatment temperature was set to 450° C.; and a 50-nm-thick siliconoxynitride film (corresponding to P1 in FIG. 46) was formed over theoxide semiconductor film under conditions where the film formationpressure and the film formation temperature were lower than those forthe sample 7.

The silicon oxynitride film (P1) included in the comparative sample 6was formed using conditions similar to those for the silicon oxynitridefilm included in the comparative sample 4.

Further, a comparative sample 7 was formed in such a manner that, in theheat treatment performed after the formation of the oxide semiconductorfilm (corresponding to OS in FIG. 46) of the comparative sample 6, theheat treatment temperature was set to 350° C.

<SIMS Measurement>

Next, SIMS measurement was performed on the sample 7 and the comparativesamples 6 and 7 to measure the hydrogen concentrations contained in theoxide semiconductor film (OS) and the silicon oxynitride film (P1). Thehydrogen concentration in the oxide semiconductor film (OS) in each ofthe samples is shown in the upper part of FIG. 46. The hydrogenconcentration in the silicon oxynitride film (P1) in each of the samplesis shown in the lower part of FIG. 46.

Note that here, SIMS measurement was performed on the sample before andafter heat treatment to determine change in hydrogen concentration dueto heat treatment. In FIG. 46, a dashed line indicates the result ofmeasurement before heat treatment, and a solid line indicates the resultof measurement after heat treatment at 350° C.

First, the hydrogen concentrations in the oxide semiconductor films (OS)are compared. The hydrogen concentrations of the sample 7 and thecomparative sample 7 which were heated at 350° C. in the heat treatmentafter the formation of the oxide semiconductor films (OS) are higherthan hydrogen concentration of the comparative sample 6. However, whencompared with the comparative sample 7, the sample 7 has a low hydrogenconcentration in the oxide semiconductor film, particularly in a regionof the oxide semiconductor film on the silicon oxynitride film (P1)side. Further, it is shown that the hydrogen concentration is reduced byperforming heat treatment after the formation of the silicon oxynitridefilm (P2).

Thus, by using the conditions for forming the oxide insulating film 23described in Embodiment 1 in a manner similar to that of the siliconoxynitride film (P1) included in the sample 7, a dense siliconoxynitride film is formed, and a hydrogen blocking effect is obtained.As a result, hydrogen contained in the silicon oxynitride film (P2) isless likely to be moved to the oxide semiconductor film even when heattreatment is performed after the formation of the silicon oxynitridefilm (P2).

Next, the hydrogen concentrations in the silicon oxynitride films (P1)are compared. The hydrogen concentration of the sample 7 is lower thanthe hydrogen concentrations of the comparative samples 6 and 7. Thus, byusing the conditions for forming the oxide insulating film 23 describedin Embodiment 1, a silicon oxynitride film with a low hydrogenconcentration can be formed.

These results show that, when the oxide insulating film is formed overthe oxide semiconductor film using the conditions for forming the oxideinsulating film 23 described in Embodiment 1, the hydrogen concentrationin the oxide semiconductor film can be reduced and an oxide insulatingfilm with a low hydrogen concentration can be formed without performingheat treatment after the oxide semiconductor film is formed. As aresult, generation of carriers in the oxide semiconductor film can bereduced, and a transistor having excellent electrical characteristics inwhich the threshold voltage is less changed can be manufactured.

Next, in a sample in which the oxide insulating film 23 described inEmbodiment 1 is formed without performing heat treatment after theformation of the oxide semiconductor film (OS), the hydrogenconcentrations in the oxide semiconductor film and the oxide insulatingfilm was measured. The results of the measurement are described below.

First of all, a process for manufacturing the sample is described.

After a 200-nm-thick silicon oxynitride film (SiON) was formed on asilicon wafer, a 100-nm-thick oxide semiconductor film (OS) was formedover the silicon oxynitride film (SiON) by a sputtering method.

Here, a silicon oxynitride film (SiON) was formed under conditionssimilar to those for the gate insulating film 17 included in the sample1 in Example 1. Further, an oxide semiconductor film (OS) was formedunder conditions similar to those for the oxide semiconductor film (S1)included in the sample 1 in Example 1.

Next, a 50-nm-thick silicon oxynitride film (P1) was formed under theconditions for forming the oxide insulating film 23 in Embodiment 1without performing heat treatment, and then, a 400-nm-thick siliconoxynitride film (P2) was formed under the conditions for forming theoxide insulating film 24 in Embodiment 1.

Here, a silicon oxynitride film (P1) was formed under conditions similarto those for the oxide insulating film 23 included in the sample 1 inExample 1. Further, a silicon oxynitride film (P2) was formed underconditions similar to those for the oxide insulating film 24 included inthe sample 1 in Example 1 (i.e. the film formation temperature was 350°C.).

Through the above process, the sample 8 was formed.

A comparative sample 8 was formed in such a manner that, instead of thesilicon oxynitride film (P1) in the sample 8, a silicon oxynitride film(P1) was formed under conditions similar to those for forming thesilicon oxynitride film included in the comparative sample 4 in Example2 (i.e., the film formation temperature was 220° C.).

<SIMS Measurement>

Next, SIMS measurement was performed on the sample 8 and the comparativesample 8 to measure the hydrogen concentrations in the oxidesemiconductor film (OS) and the silicon oxynitride film (P1).

Further, SIMS measurement was performed on the sample 8 and thecomparative sample 8 after heat treatment was performed at 350° C. in anatmosphere containing nitrogen and oxygen for one hour.

FIGS. 50A and 50B show comparison between the H concentrations beforeand after heat treatment in the oxide semiconductor films (OS) of thesample 8 and the comparative sample 8. FIGS. 51A and 51B show comparisonbetween H concentrations before and after heat treatment in the siliconoxynitride films (P1) of the sample 8 and the comparative sample 8. InFIGS. 50A and 51A, the H concentrations in the sample 8 and thecomparative sample 8 before heat treatment are shown. In FIGS. 50B and51B, the H concentrations in the sample 8 and the comparative sample 8after heat treatment are shown. Note that in FIGS. 50A and 50B and FIGS.51A and 51B, thick solid lines indicate measurement results of thesample 8, and thin solid lines indicate measurement results of thecomparative sample 8.

It is shown from FIGS. 50A and 50B that, in the oxide semiconductor film(OS), hydrogen concentration of the sample 8 whose silicon oxynitridefilm (P1) was formed at 350° C. is lower than that of the comparativesample 8 whose silicon oxynitride film (P1) was formed at 220° C.

It is shown from FIGS. 51A and 51B that, in the silicon oxynitride film(P1), hydrogen concentration of the sample 8 whose silicon oxynitridefilm (P1) was formed at 350° C. is lower than that of the comparativesample 8 whose silicon oxynitride film (P1) was formed at 220° C.

In terms of the hydrogen concentration, FIGS. 50A and 51A show the sameresult, and FIGS. 50B and 51B show the same result. That is, the sameresults can be obtained whether or not heat treatment is performed afterthe formation of the silicon oxynitride film (P2).

These results show that, when the silicon oxynitride film is formed at350° C., the hydrogen concentration in the oxide semiconductor film canbe reduced without performing heat treatment after the oxidesemiconductor film is formed.

Example 4

In Example 4, the relation between plasma treatment performed on asurface of the oxide semiconductor film and the hydrogen concentrationin the oxide semiconductor film is described with reference to FIGS. 52Aand 52B.

First of all, a process for manufacturing a sample is described.

A 35-nm-thick oxide semiconductor film was formed over a quartzsubstrate by a sputtering method. Next, the oxide semiconductor film wasexposed to oxygen plasma generated in a dinitrogen monoxide atmosphere.

The oxide semiconductor film was formed in such a manner that asputtering target where In:Ga:Zn=1:1:1 (atomic ratio) was used, argonwith a flow rate of 100 sccm and oxygen with a flow rate of 100 sccmwere supplied as the sputtering gas into a treatment chamber of thesputtering apparatus, the pressure in the treatment chamber wascontrolled to 0.6 Pa, and a direct-current power of 3 kW was supplied.Note that the oxide semiconductor film was formed at a substratetemperature of 200° C.

Next, oxygen plasma was generated in such a manner that dinitrogenmonoxide with a flow rate of 10000 sccm was supplied to the treatmentchamber of the plasma CVD apparatus, the pressure in the treatmentchamber was controlled to 200 Pa, and a DC power of 150 W was supplied.Further, the oxide semiconductor film was exposed to the oxygen plasmafor 300 seconds. The substrate temperature at this time was 350° C.

Through the above process, a sample 9 was formed.

A comparative sample 9 was formed in such a manner that oxygen plasmatreatment was not performed in the sample 9.

A comparative sample 10 was formed in such a manner that, instead of theoxygen plasma treatment, heat treatment was performed in a vacuumatmosphere in the sample 9.

In the comparative sample 10, nitrogen with a flow rate of 10000 sccmwas supplied to a treatment chamber of a plasma CVD apparatus, thepressure in the treatment chamber was controlled to 175 Pa, thesubstrate temperature was 350° C., and heat treatment was performed for600 seconds.

<TDS Measurement>

Next, TDS measurement was performed on the sample 9, the comparativesample 9, and the comparative sample 10. FIGS. 52A and 52B show themeasurement results. FIG. 52A is a chart showing the results of TDSmeasurement performed on the sample 9, the comparative sample 9, and thecomparative sample 10. FIG. 52B is a chart where part of FIG. 52A (therange of 2×10⁻¹¹ to 6×10⁻¹¹ on the vertical axis indicating intensity inFIG. 52A) is enlarged. In FIGS. 52A and 52B, thick solid lines indicateTDS measurement results of the sample 9, dashed lines indicate TDSmeasurement results of the comparative sample 9, and thin solid linesindicate TDS measurement results of the comparative sample 10. In FIGS.52A and 52B, the vertical axis represents intensity corresponding to theamount of released water, and the horizontal axis represents heattreatment temperature.

It is shown from FIGS. 52A and 52B that, at a temperature of 100° C. andat temperatures from 250° C. to 370° C., the amount of water releasedfrom the sample 9 is smaller than that released from the comparativesample 9. It is also shown that, at temperatures from 250° C. to 400°C., the amount of water released from the sample 9 is smaller than thatreleased from the comparative sample 10.

Thus, when the oxide semiconductor film is exposed to oxygen plasmagenerated in a dinitrogen monoxide atmosphere, the amount of waterreleased from the oxide semiconductor film can be reduced. This isprobably because, by exposure of the oxide semiconductor film to oxygenplasma generated in a dinitrogen monoxide, hydrogen contained in theoxide semiconductor film reacts with oxygen in the oxygen plasma, andthus, water is formed and released.

Example 5

Example 5 describes the relation between whether or not plasma treatmentis performed on the surface of the oxide semiconductor film and theVg-Id characteristics of the transistor.

First of all, a manufacturing process of a transistor included in asample 10 is described. In this example, the process is described withreference to FIGS. 2A to 2D and Example 1.

As shown in FIG. 2A, a glass substrate was used as the substrate 11, andthe gate electrode 15 was formed over the substrate 11 under conditionssimilar to those for the sample 1 in Example 1.

Next, the gate insulating film 17 was formed over the gate electrode 15under conditions similar to those for the sample 2 in Example 1.

Next, the oxide semiconductor film 18 was formed to overlap with thegate electrode 15 with the gate insulating film 17 providedtherebetween.

Here, a 35-nm-thick oxide semiconductor film was formed over the gateinsulating film 17 by a sputtering method. Then, a mask was formed overthe oxide semiconductor film by a photolithography process, and theoxide semiconductor film was partly etched using the mask to form theoxide semiconductor film 18.

Note that the oxide semiconductor film was formed under the followingconditions: a sputtering target where In:Ga:Zn=1:1:1 (atomic ratio) wasused; argon with a flow rate of 60 sccm and oxygen with a flow rate of140 sccm were supplied as a sputtering gas into the treatment chamber ofthe sputtering apparatus; the pressure in the treatment chamber wascontrolled to 0.6 Pa; and a direct-current power of 3 kW was supplied.Note that the oxide semiconductor film was formed at a substratetemperature of 200° C.

For the structure obtained through the steps up to here, FIG. 2B can bereferred to.

Next, the gate electrode was exposed by partly etching the gateinsulating film 17 (not illustrated). Then, as shown in FIG. 2C, thepair of electrodes 21 and 22 in contact with the oxide semiconductorfilm 18 was formed under conditions similar to those for the sample 1 inExample 1. After that, a surface of the oxide semiconductor film wassubjected to cleaning treatment using a phosphoric acid solution inwhich 85% phosphoric acid was diluted by 100 times.

Next, without exposure of the oxide semiconductor film 18 to oxygenplasma, the oxide insulating film 23 and the oxide insulating film 24were formed over the oxide semiconductor film 18 and the pair ofelectrodes 21 and 22 in a manner similar to that of the sample 1 inExample 1.

Next, heat treatment was performed under conditions similar to those forthe sample 1 in Example 1 to release water, nitrogen, hydrogen, or thelike from the oxide insulating film 23 and the oxide insulating film 24and supply part of oxygen contained in the oxide insulating film 24 tothe oxide semiconductor film 18.

Next, the nitride insulating film 25 was formed under conditions similarto those for the sample 2 in Example 1 (see FIG. 2D).

Next, with the use of conditions similar to those for the sample 1 inExample 1, the following were performed: formation of the opening forexposing part of the pair of electrodes 21 and 22; formation of theplanarization film; formation of the conductive film connected to partof the pair of electrodes; and heat treatment. Thus, the sample 10having a transistor was formed.

A sample 11 was formed in the following manner: in the sample 10, thepair of electrodes 21 and 22 was formed, and after the surface of theoxide semiconductor film was subjected to cleaning treatment using aphosphoric acid solution in which 85% phosphoric acid was diluted by 100times, the oxide semiconductor film was exposed to oxygen plasmagenerated in a dinitrogen monoxide atmosphere.

The conditions for generating oxygen plasma were similar to those forthe sample 9 described in Example 4. Further, the oxide semiconductorfilm was exposed to the oxygen plasma for 300 seconds. The substratetemperature at this time was 350° C.

A transistor included in each sample has a channel length (L) of 6 μmand a channel width (W) of 50 μm.

<Vg-Id Characteristics>

Next, initial Vg-Id characteristics of the transistors included in thesample 10 and the sample 11 were measured. Here, change incharacteristics of current flowing between a source electrode and adrain electrode (hereinafter referred to as the drain current), that is,Vg-Id characteristics were measured under the following conditions: thesubstrate temperature was 25° C., the potential difference between thesource electrode and the drain electrode (hereinafter referred to as thedrain voltage) was 1 V or 10 V, and the potential difference between thesource electrode and the gate electrode (hereinafter referred to as thegate voltage) were changed from −15 V to +20 V.

FIGS. 53A and 53B show the Vg-Id characteristics of the transistorsincluded in the samples. In each graph in FIGS. 53A and 53B, thehorizontal axis indicates gate voltage Vg, the left vertical axisindicates drain current Id, and the right vertical axis indicatesfield-effect mobility. Further, the solid lines indicate the Vg-Idcharacteristics at the drain voltages Vd of 1 V and 10 V, and the dashedlines indicate the field-effect mobility with respect to the gatevoltages at the drain voltage Vd of 10 V. Note that the field effectmobility was obtained by operation of each sample in a saturationregion.

Further, in each of the samples, 20 transistors having the samestructure were formed on the substrate.

It is shown from FIGS. 53A and 53B that the samples 10 and 11 havefavorable switching characteristics. Further, the threshold voltages ofthe sample 11 are shifted in the positive direction as compared withthose of the sample 10. From the results shown in FIGS. 53A and 53B andthe results of measuring the amount of released water by TDS in Example4, the following is suggested: by exposure of the surface of the oxidesemiconductor film to oxygen plasma generated in a dinitrogen monoxideatmosphere, the amount of oxygen supplied to the oxide semiconductorfilm is increased, and water contained in the oxide semiconductor filmis released, whereby the transistor can have more excellent Vg-Idcharacteristics.

Note that when the oxide insulating film 23 was formed by a plasma CVDmethod, part of dinitrogen monoxide in a source gas became an oxygenradical in plasma and the radical was supplied to the oxidesemiconductor film. Thus, the sample 10 has excellent Vg-Idcharacteristics without plasma treatment for exposure of the surface ofthe oxide semiconductor film to oxygen plasma generated in a dinitrogenmonoxide atmosphere.

This application is based on Japanese Patent Application serial no.2013-008628 filed with Japan Patent Office on Jan. 21, 2013, andJapanese Patent Application serial no. 2013-053192 filed with JapanPatent Office on Mar. 15, 2013, the entire contents of which are herebyincorporated by reference.

What is claimed is:
 1. A method for manufacturing a semiconductordevice, comprising the steps of: forming a first gate electrode over asubstrate; forming a gate insulating film over the first gate electrode;forming an oxide semiconductor film over the gate insulating film;forming a pair of electrodes electrically connected to the oxidesemiconductor film without performing heat treatment after the oxidesemiconductor film is formed; forming a first oxide insulating film overthe oxide semiconductor film by a plasma CVD, wherein a film formationtemperature of the first oxide insulating film is higher than or equalto 280° C. and lower than or equal to 400° C.; forming a second oxideinsulating film over the first oxide insulating film; and performingheat treatment at a temperature higher than or equal to 150° C. andlower than or equal to 400° C.
 2. The method for manufacturing asemiconductor device according to claim 1, wherein a treatment chamberis evacuated and a pressure in the treatment chamber is set to begreater than or equal to 100 Pa and less than or equal to 250 Pa byintroducing a source gas into the treatment chamber, and wherein thefirst oxide insulating film is formed in the treatment chamber bysupplying a high-frequency power to an electrode provided in thetreatment chamber.
 3. The method for manufacturing a semiconductordevice according to claim 1, wherein a treatment chamber is evacuatedand a pressure in the treatment chamber is set to be greater than orequal to 100 Pa and less than or equal to 250 Pa by introducing a sourcegas into the treatment chamber, wherein the second oxide insulating filmis formed in the treatment chamber by supplying a high-frequency powerhigher than or equal to 0.17 W/cm² and lower than or equal to 0.5 W/cm²to an electrode provided in the treatment chamber, and wherein atemperature of the substrate placed in the treatment chamber is atemperature higher than or equal to 180° C. and lower than or equal to280° C.
 4. The method for manufacturing a semiconductor device accordingto claim 1, wherein, as the first oxide insulating film and the secondoxide insulating film, a silicon oxide film or a silicon oxynitride filmis formed using a deposition gas containing silicon and an oxidizing gasas a source gas.
 5. The method for manufacturing a semiconductor deviceaccording to claim 1, wherein, as the first oxide insulating film andthe second oxide insulating film, a silicon oxynitride film is formedusing silane and dinitrogen monoxide as a source gas.
 6. The method formanufacturing a semiconductor device according to claim 1, wherein theoxide semiconductor film comprises at least one of In and Ga.
 7. Themethod for manufacturing a semiconductor device according to claim 1further comprising: forming a nitride insulating film over the secondoxide insulating film.
 8. The method for manufacturing a semiconductordevice according to claim 7 further comprising: forming a second gateelectrode over the nitride insulating film.
 9. A method formanufacturing a semiconductor device, comprising the steps of: forming agate electrode over a substrate; forming a gate insulating film over thegate electrode; forming an oxide semiconductor film over the gateinsulating film; forming an oxide film over the oxide semiconductorfilm; forming a pair of electrodes electrically connected to the oxidesemiconductor film over the oxide film, without performing heattreatment after the oxide film is formed; forming a first oxideinsulating film over the oxide film and the pair of electrodes by aplasma CVD, wherein a film formation temperature of the first oxideinsulating film is higher than or equal to 280° C. and lower than orequal to 400° C.; forming a second oxide insulating film over the firstoxide insulating film; and performing heat treatment at a temperaturehigher than or equal to 150° C. and lower than or equal to 400° C. 10.The method for manufacturing a semiconductor device according to claim9, wherein a treatment chamber is evacuated and a pressure in thetreatment chamber is set to be greater than or equal to 100 Pa and lessthan or equal to 250 Pa by introducing a source gas into the treatmentchamber, and wherein the first oxide insulating film is formed in thetreatment chamber by supplying a high-frequency power to an electrodeprovided in the treatment chamber.
 11. The method for manufacturing asemiconductor device according to claim 9, wherein a treatment chamberis evacuated and a pressure in the treatment chamber is set to begreater than or equal to 100 Pa and less than or equal to 250 Pa byintroducing a source gas into the treatment chamber, wherein the secondoxide insulating film is formed in the treatment chamber by supplying ahigh-frequency power higher than or equal to 0.17 W/cm² and lower thanor equal to 0.5 W/cm² to an electrode provided in the treatment chamber,and wherein a temperature of the substrate placed in the treatmentchamber is a temperature higher than or equal to 180° C. and lower thanor equal to 280° C.
 12. The method for manufacturing a semiconductordevice according to claim 9, wherein, as the first oxide insulating filmand the second oxide insulating film, a silicon oxide film or a siliconoxynitride film is formed using a deposition gas containing silicon andan oxidizing gas as a source gas.
 13. The method for manufacturing asemiconductor device according to claim 9, wherein, as the first oxideinsulating film and the second oxide insulating film, a siliconoxynitride film is formed using silane and dinitrogen monoxide as asource gas.
 14. The method for manufacturing a semiconductor deviceaccording to claim 9, wherein the oxide semiconductor film comprises atleast one of In and Ga.
 15. The method for manufacturing a semiconductordevice according to claim 9, wherein the oxide film comprises at leastone of elements included in the oxide semiconductor film.
 16. The methodfor manufacturing a semiconductor device according to claim 9, wherein aconduction band bottom of the oxide film is closer to a vacuum levelthan a conduction band bottom of the oxide semiconductor film is. 17.The method for manufacturing a semiconductor device according to claim 9further comprising: forming a nitride insulating film over the secondoxide insulating film.